I'm very much aware asking technical questions is against the rules, sorry.
Here's the deal, I wanted to divide my premade rtk deck into smaller branching groups like what you can see here in pic related:

The idea is to make a deck like Memrise and have all kanji divided into parts of 20.
Anki's user manual only explains only to make a new deck into a cascading tree like that, NOT how to turn an already existing deck with 3000+ cards into folding tree like that.
Anyway, sorry for breaking the rules.

Just drag one deck onto the other to make it a child deck.

btw, there's an anki small question thread.