I was reading the D&D Beyond about wild magic sorcerer and I was thinking about something when I read the recommended feats, in particular the Ritual Caster Feat.
Is it possible for a sorcerer to write all the ritual spells they know in the ritual book of Ritual Caster Feat when they acquire this feat? Or do they have to find the written version somewhere?
In the feat description, it looks like no, but I feel like it doesn't make sense, since the sorcerer knows the spells.
Unfortunately, the Ritual Caster feat (PHB, p. 169) has only two ways to put spells in the book.
First is when you select the feat:
When you choose this feat, you acquire a ritual book holding two 1st-level spells of your choice.
So those first two spells just automatically show up in the book. The next bullet point describes how to add new spells:
If you come across a spell in written form, such as a magical spell scroll or a wizard's spellbook, you might be able to add it to your ritual book.
This allows you to transcribe a written spell, either from a scroll or spellbook, into your ritual book.
There is no mechanic for transferring a 'spell known', but not written, into the ritual book. Unless your DM allows creation of spell scrolls by PCs (which they could using the "Scribing a Spell Scroll" (XGtE, p. 133) or the "Crafting a Magic Item" (DMG, p. 128) downtime activity options), then there is no way to transfer a known spell from a Sorcerer(or any other spellcaster) into the spellbook without first putting it in written form.
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.
