The SCX2 mod added non-player units, and the SCU mod made some playable. You'd have to investigate how it works and replicate the installation method. Or maybe ask the ones that were part of it. I do not know how.
See at the bottom of this Subguru.com page for a download link:
http://subguru.com/downloads.html
(it might not work if you have the Naval Combat Pack version)