#No, there are no mechanics provided for a player character to become an "otherworldly patron"

The beings that serve as patrons for warlocks are mighty inhabitants of other planes of existence—not gods, but almost godlike in their power. Various patrons give their warlocks access to different powers and invocations, and expect significant favors in return.

 

Some patrons collect warlocks, doling out mystic knowledge relatively freely or boasting of their ability to bind mortals to their will. Other patrons bestow their power only grudgingly, and might make a pact with only one warlock. Warlocks who serve the same patron might view each other as allies, siblings, or rivals.

A warlock is defined by a pact with an otherworldly being. Sometimes the relationship between warlock and patron is like that of a cleric and a deity, though the beings that serve as patrons for warlocks are not gods. A warlock might lead a cult dedicated to a demon prince, an archdevil, or an utterly alien entity—beings not typically served by clerics. More often, though, the arrangement is similar to that between a master and an apprentice. The warlock learns and grows in power, at the cost of occasional services performed on the patron’s behalf.

 

[...]

 

Stories of warlocks binding themselves to fiends are widely known. But many warlocks serve patrons that are not fiendish. Sometimes a traveler in the wilds comes to a strangely beautiful tower, meets its fey lord or lady, and stumbles into a pact without being fully aware of it. And sometimes, while poring over tomes of forbidden lore, a brilliant but crazed student’s mind is opened to realities beyond the material world and to the alien beings that dwell in the outer void.

 

[...]

 

As you make your warlock character, spend some time thinking about your patron and the obligations that your pact imposes upon you. What led you to make the pact, and how did you make contact with your patron? Were you seduced into summoning a devil, or did you seek out the ritual that would allow you to make contact with an alien elder god? Did you search for your patron, or did your patron find and choose you? Do you chafe under the obligations of your pact or serve joyfully in anticipation of the rewards promised to you?

 

Work with your DM to determine how big a part your pact will play in your character’s adventuring career. Your patron’s demands might drive you into adventures, or they might consist entirely of small favors you can do between adventures.

 

What kind of relationship do you have with your patron? Is it friendly, antagonistic, uneasy, or romantic? How important does your patron consider you to be? What part do you play in your patron’s plans? Do you know other servants of your patron?

No, there are no mechanics provided for a player character to become an "otherworldly patron"

The beings that serve as patrons for warlocks are mighty inhabitants of other planes of existence—not gods, but almost godlike in their power. Various patrons give their warlocks access to different powers and invocations, and expect significant favors in return.

Some patrons collect warlocks, doling out mystic knowledge relatively freely or boasting of their ability to bind mortals to their will. Other patrons bestow their power only grudgingly, and might make a pact with only one warlock. Warlocks who serve the same patron might view each other as allies, siblings, or rivals.

A warlock is defined by a pact with an otherworldly being. Sometimes the relationship between warlock and patron is like that of a cleric and a deity, though the beings that serve as patrons for warlocks are not gods. A warlock might lead a cult dedicated to a demon prince, an archdevil, or an utterly alien entity—beings not typically served by clerics. More often, though, the arrangement is similar to that between a master and an apprentice. The warlock learns and grows in power, at the cost of occasional services performed on the patron’s behalf.

[...]

Stories of warlocks binding themselves to fiends are widely known. But many warlocks serve patrons that are not fiendish. Sometimes a traveler in the wilds comes to a strangely beautiful tower, meets its fey lord or lady, and stumbles into a pact without being fully aware of it. And sometimes, while poring over tomes of forbidden lore, a brilliant but crazed student’s mind is opened to realities beyond the material world and to the alien beings that dwell in the outer void.

[...]

As you make your warlock character, spend some time thinking about your patron and the obligations that your pact imposes upon you. What led you to make the pact, and how did you make contact with your patron? Were you seduced into summoning a devil, or did you seek out the ritual that would allow you to make contact with an alien elder god? Did you search for your patron, or did your patron find and choose you? Do you chafe under the obligations of your pact or serve joyfully in anticipation of the rewards promised to you?

Work with your DM to determine how big a part your pact will play in your character’s adventuring career. Your patron’s demands might drive you into adventures, or they might consist entirely of small favors you can do between adventures.

What kind of relationship do you have with your patron? Is it friendly, antagonistic, uneasy, or romantic? How important does your patron consider you to be? What part do you play in your patron’s plans? Do you know other servants of your patron?

Ultimately, though, once they're at the point at which they're equal in power to their patron... Well, there's no more knowledge to learn; no greater power to gain. And it makes no sense to make a pact with yourself to get a portion of your own power and knowledge.

Once you're virtually a god, there's not really much higher to go as a PC - certainly nowhere that D&D 5e is designed to handle as a system. It can make for a cool story beat, but that's about it.

Ultimately, though, once they're at the point at which they're equal in power to their patron... Well, there's no more knowledge to learn; no greater power to gain. (And it makes no sense to make a pact with yourself to get a portion of your own power and knowledge.)

Once you're virtually a god, there's not really much higher to go as a PC - certainly nowhere that D&D 5e is designed to handle as a system. It can make for a cool story beat, but that's about it.

Even from the DM perspective of running a patron "NPC", no such detailed mechanics are provided about the patron-warlock relationship. Besides the broad guidelines I've referenced above, it's basically up to the DM to determine such details - no particular mechanics are described in any official 5e materials. In short, the rules leave it to the DM to work out with their player.