I understand that there is no Admiral uniform for when you get to admiral. You'll see NPCs wear their uniforms admiral uniforms, like Admiral Quinn, where it is a variation with a longer top and a flap on the chest. It's similar to the WOK Uniforms.
I'd like to see this added in the future, since it's obviously something that people would like.
I do believe an Admiral's uniform exclusive to RAs and above is a must. As a RA5 I would like to have a uniform that distinguishes me from lower officers a symbol for the work I put in to become an Admiral. Personally I want the one Quinn and the other NPC admirals ware. Since the outfit is already in the game it would be relativilly easy to make it a player option. This is one of those items that people have been asking for since the beginning, since it would be so easy to implement I am kinda puzzled as to why they have not done it yet.