This uniform is unlocked when you launch STO via the Arc Launcher once on PC, or for Playstation Plus users.
List of Pieces
The Khan Outfit outfit contains pieces for both upper and lower parts. The pieces can be found in the "Off Duty" section of the Tailor.
Upper: Khan's Tunic
Wrist Attach Left: Khan's Wristband
Collar: Khan's Pendant
Feet: Boot Khan Off-Duty
Belt: Khan Off-Duty