So, as mentioned in the title Can you add (fits) to a clothing on the ground? (mapgen, items) or spawn it with (fits) from wish for an item?
There’s no function to specifically ask for it, if you just spawn a bunch of an article of clothing it might occasionally give you a fitted one since it uses the normal spawning code. You can also just bump your tailoring and spawn the materials to fit it.
Does this work with “wished for” items as well?
That’s what you asked and what Kevin answered. Writing fitted clothing into mapgen (forex, clothes in a Your House ought to fit you) would be nifty but I don’t think we support that yet.