Couple things:
There's gotta be a file that your front end is using that knows what your favorites are. If that file just happens to be an XML file, based off of MAME's xml output, you can import that file directly into romlister and then export it to another front end from there.
If there's no XML file of your favorites, then if there was any way to get your list exported as a standard rom name list in a text file, I've been wanting to add a text import utility into romlister. I'd be happy to work with you on this, but you're going to need to find a way to export it somehow for me to work on the import side of it.
PM me and we'll work out the details.