Raise your relationship with the Confederate Renegades. When high enough, you should be able to get troops from their parties when you speak with them.
Raise the relationship by helping them in battles. Any battle helps, even if against US peasants.
Lately I've been experimenting with changing faction relationships and party sizes so there are more battles going on and more opportunities to gain or lose rep with various factions -- a much better long, term solution. For example, I made Confederates enemies with all outlaws, the Union Army, Indian Raiders, and every other faction that seemed remotely likely to be hostile to them. Then I made the Confederate party sizes more variable, so sometimes you only get a handful and sometimes you get a lot. That way you're more likely to find smaller groups of them being attacked and larger groups attacking. So far the results are promising.