Thanks for asking!
For a non-profit mod for another game, I'm generally ok with it if someone asks first. Of course, I reserve the right to say no if the mod is objectionable, or for any reason whatsoever, really, and to withdraw permission at any time for any reason.
But, in this particular case: go for it! Provided that there's nothing in the other game's EULA that would, say, assign them rights for any modded content, or something like that, that would require you to have rights to the content you're using for the mod. I doubt that's the case, but thought I'd mention it just to be on the safe side.
(And, naturally, for a commercial project, the answer would always be a firm "no". Although, that probably doesn't need saying.)