time to pull out the geek card… In the later episodes of DS9, Nog, a Ferengi applied to be in the Academy. Nog and Sisko informed us that in order for a member of a non-Federation species to be considered, they have to be recommended by a command-level officer. While we’ve never seen any attempts by any Federation enemies to join Starfleet, I’m willing to bet the same sort of thing would apply. That there is some sort of “not unless <X>” rule. Put more generally, it’s a generic rule of life that you can do anything the rules tell you you can’t, so long as you have the right signatures.