Hi! I'm new to modding but wanted to play around with some of the tools and found myself stuck.

I want one of my SIMs to want one of the available 'fears'. I tried setting the fear in the want category in the SIM editor, but that seemed to just break the game and it crashed. Does anyone know how to change a Fear into a Want or make the Fears accessible to the 'Wants' functions?

Like if a fear is "getting fat", but isn't available as a Want, can I mod the game so that "getting fat" could be something they could Want?

Well, you probably can change a fear into a want, but it may have a funny effect on sims' wants going forward. Because want trees that control logic/continuity/consistency of wants and fears expect fulfilled fears to be a bad thing, this may lead to unexpected stuff, not game breaking, but unexpected

Your SimPE probably corrupted the file, that's why the game crashed. To edit or create wants you have to use very old version of SimPE or manually change encoding of the XWNT files (extract -> edit in code editor -> import back). That's what I did with my LTWs

It's also possible to edit want trees, of course