The sorcerer doesn't know any ritual spells
That is not to say that spells on their spell list do not contain the ritual tag, but that all the spells that they know, they know innately.
An event in your past, or in the life of a parent or ancestor, left an indelible mark on you, infusing you with arcane magic. This font of magic, whatever its origin, fuels your spells.
Therefore, they do not know how to convert from "font magic" to making it a ritual. The same can be said for Warlocks. Their spell casting ability is granted from their patron.
This was covered in Sage Advice:
Q: Can [a warlock with a Book of Shadows] write down a spell they have prepared already?
A: no - requires creation of a scroll or spellbook
There is a workaround mentioned in this question:
If your DM allows scroll scribing/magic item crafting, you could create a scroll and then copy it into you Book of Secrets.
It's covered in Xanathar's: (Link works only if you own Xanathar's)
Scribing a spell scroll takes an amount of time and money related to the level of the spell the character wants to scribe, as shown in the Spell Scroll Costs table. In addition, the character must have proficiency in the Arcana skill and must provide any material components required for the casting of the spell. Moreover, the character must have the spell prepared, or it must be among the character's known spells, in order to scribe a scroll of that spell.
So as long as you have time, money, and the Arcana proficiency, you might be able to convert it.