You’ll want to head to either the FEBuilder GBA or Skill System and associated threads:
From there you’ll have a lot of questions and using the search feature – Search results for '' - Fire Emblem Universe – will probably be able to help but it might not, at which point you’ll want to ask your question either in a new topic or in the relevant channel in the discord -
To answer your question more directly, however, if you want to add Staves to the Falcoknight, just doing so will cause their Sword/Lance ranks to be hidden, which is fixed with multiple variations of this patch -
In FEBuilder’s Advanced Editor, there’s a fairly intuitive “Search” feature that will return additional results at the bottom of the program for you to browse the patches list - It might be called “melee and magic” or “weapon rank” and you’ll just have to try things until you find what people’ve decided it should be called (a rather unfortunate thing that happens when everyone gets to name things on their own).
In a Skill System based buildfile, there’s an area of Engine Hacks called Modular Stat Screen that handles the same feature in this thread’s hack of “Display only weapon ranks you actually have”.
The hack in this thread can be found here in the unified hacking dropbox: weapon ranks display hack; but you might want extended weapon ranks but I can’t actually remember which one does what off the top of my head.