Max out Romulan Rep, which makes your character an "expert" on Romulans. Max out Espionage DOff category.
Get the option for a species respec to become a Romulan, via surgical alteration.
Get extra assignments at the Embassy to help your fleet if you're a Romulan, a daily awarding random items needed for the fleet holding.
It makes more sense than Romulans actually joining. Instead, you're doing what Kirk and Picard did and getting surgically altered INTO a Romulan. You just stay that way until they add species respecs and possibly more "surgical alteration" options in game.