Why wouldn't it be. After all the Ferengi are not part of UFP, but Nog went to Starfleet Academy.
Ensign Ro was already in Starfleet even though Bajor wasn't part of the UFP (at the time). So, being part of the UFP has never really
been a requirement for acceptance to Starfleet.
I think that's kind of cool actually. As long as Starfleet Command are satisfied that the applicant isn't a spy for a competing political power, they'd probably accept anyone