I'm using the Joomla 2.5 "User - Profile" plug in for standard fields and adding some custom fields using a plug-in called ARRA User Migrate. Can I map JLinked fields into "User - Profile" fields?
No, we don't have integration with the built in User - Profile plugin yet for automatically importing information. It's something we've been intending to do, but honestly not had any requests for it in a long time. The User - Profile fields will show up on the Login / Register page, so your users can fill them out, so you'll still be able to get that information from your users on registration.
Hope that answers your question. I've put a note on our issue tracker item that someone is looking for this feature. It will help increase it's priority a little, but I don't have any timeline on when it'd be available.