I’d like to add casual modes to pre-existing rom hacks (not base games) but I’m not sure how? I’ve seen some threads mention that it is possible via FEBuilder but they never actually explain how. I have poked around some hacks with FEB a bit so I’m not a complete stranger to it but definitely not experienced either.
Casual Mode can be added with a patch. See here for how to install patches: UltraxBlade's #febuilder-help FAQ - #2 by UltraxBlade
There are multiple versions of casual mode available, easiest version to set up in this case would probably be the “menu at game start” version.
And this should (generally) work on most other rom hacks?
Some hacks may have other code that conflicts with it, so I make no promises, but it probably will