A warlock has no ability to create fiendish pacts with other mortals—the only fiendish pact a warlock gets to create is their own, with their own patron.
That’s about all we can say with absolute certainty—we have rules for what a warlock can and cannot do, and this isn’t listed among them. That doesn’t mean we can’t fill in some blanks here, based on other things we know.
For example, the lack of any warlock ability that allows making such pacts doesn’t have to mean that the warlock is pointless here—you call them an “agent,” and that’s exactly what they could be, in the legal sense.
An agent, in legal terminology, is a person who has been legally empowered to act on behalf of another person or an entity. An agent may be employed to represent a client in negotiations and other dealings with third parties. […] The person represented by the agent in these scenarios is called the principal.
(“agent” in Investopedia)
So here, the warlock could be the patron’s agent, which means that the warlock can negotiate deals on the behalf of the principal, the patron. That way, a warlock can offer great and wondrous power,™ in exchange for committing one’s soul to Hell, even though they neither have the ability to grant great and wondrous power,™ nor the ability to collect souls. What they are doing is negotiating on their patron’s behalf, and the patron is the one granting the power and collecting the soul. This may, or may not, require the patron’s involvement—that’s up to the patron, how much authority they give the warlock to act unilaterally.
So things can range from the warlock just introducing the third party to the patron, up to the warlock having been given some magic contract that they are able to sign in the patron’s name, binding the patron and the third party together. One expects lower-level, less trusted warlocks to be more of the former kind of thing, while higher-level warlocks might be entrusted with the latter ability. This kind of thing always demands a cost–benefit analysis for the patron—the more independent the warlock is, the more effective they’ll be as an agent, and the less hassle the patron will have collecting those souls, but at the same time, if the agent has been given the authority to sign in the principal’s name, a warlock could potentially sign a contract that the patron does not want, and the patron will be bound by it. Many fiends, no doubt, take a route followed by many people in the business world—the agent can handle all the negotiation and everything right up to the final signature, but that requires the principal’s approval.
Anyway, in all cases, the third party who signs on to a contract is now eligible to take levels of the warlock class with a fiendish patron. (The rules don’t make this a “requirement,” and PCs can be “special” anyway, but then the rules leave a lot of the how and why of leveling up undefined and up to the table.) If you, as DM, like, you can even allow the third party to immediately level up, gaining that 1st level of warlock.¹ This may, however, be a premium feature that the patron doesn’t offer to just anybody. In extreme cases, the patron may even be able to offer several warlock levels all at once—I had a game where a PC died and the player wanted to play as a particular NPC, but that NPC had no levels, so what we did was that they signed a fiendish pact to suddenly get a bunch of warlock levels to make them match up with the rest of the party. We didn’t have a warlock-agent involved, but we could have.
As for the soul, the collection of the soul happens after the death of the third party. That’s the usual nature of a Faustian bargain, after all: enjoy temporal power and prestige, in exchange for an eternity of damnation and suffering.
- Be careful allowing this with PCs, though—if you do, I recommend delaying the new warlock’s next level-up to coincide with the rest of the party reaching the same level, i.e. the warlock is only ahead temporarily