Worf was a Federation citizen and also a Starfleet Officer.

Also we can play as a Klingon in Federation side, its not exclusive to KDF (I do wish Feringi would also be a unlocked race for KDF).

At Tier 4 Diplomacy we do get a opposite fraction BO that keeps his or her faction uniform but so did Riker when he served for a time aboard a Klingon Bird-of-Prey.