As far as I know, your limited only by the registry you choose for the foal. If a registry outside of RPSI does not have the particular stallion approved with them, you may not be able to get papers. However, there are stallions listed with RPSI who are approved with other registries and depending on their mare requirements, you may find you'll be able to get papers other than from RPSI.
Talk to Oldenburg before you make the move.
Hyperion Stud, LLC.
Europe's Finest, Made in America WWW.HYPERIONSTUD.com
Standing Elite and Approved Stallions