Hi Danna, not entirely sure what you’re asking…
But, if you are asking if you can move a brush from, for example, the Oil Brush folder into either the root Custom Brush folder or a sub folder within it, and still be able to use said Oil Brush afterwards, then no that won’t work.
Likewise you can’t put a Custom Brush into one of the other tool categories folders and still use it.
If I’ve understood correctly, what would you hope to achieve in doing what you asked?
If you can give a bit more info, maybe there is another way to achieve your goal? (I'm thinking maybe Toolboxes here...)