nrf for Norman

Neither would work. The first would trigger an error, and in the second example the second definition will override the first.

So there's no way to get both {{wikipedia|lang=roa-nor}} and {{wikipedia|lang=nrf}} to point to w:nrm:?

Module:wikimedia languages/data defines the mapping from Wikimedia languages to Wiktionary ones. So it's what you'd use if you have a Wikimedia code and want to convert it to a local code. What you're doing is the reverse, which is handled by our regular language modules.