Originally Posted by
Magister Militum Flavius Aetius
Adding the Bulgars would be pointless. The Bulgars of the future Khan Asparuch were a subject people under the Hepthaltites in Bactria at this time. They wouldn't even gain anything more than a role as peasants for another century after the game starts.
If you're talking about the "Bulgars" like the Kutrigurs, Utigurs, etc. They're already in the game: they're called the Huns.
EDIT:
The Kutrigurs and Utigurs were created when Attila's two main sons (Dengzich and either Ernak or Ellak, can't remember which one) retreated East of the Carpathians into Wallachia and the Dniester region. Eventually both retreated past the Dniester north of the black sea where they set up the Utigur (30 Bulgars) and Kutrigur (7 Bulgars) kingdoms, just above the Saragurs (who had migrated into the region in 460) and Akatir Huns north of the Crimea. The Onogurs (10 Bulgars) took up residence between the Black sea and the Caspian, and the Sabirs were in the Derbent pass stretching north to the Volga.
All of these groups were Huns, who were the original Oghur-speaking people. The Rouran or Kokturks (not sure which) drove the Avars (a splinter of the Hepthaltites) West in the 460's which forced most of the assimilated Oghur groups in the Hunnic realm west into Europe, while the Avars took up residence in Khazakstan. In the 550's the Kokturks drove the Avars into Europe as the Hepthaltite confederation began its decline and other Turkish peoples took up residence in Khazakstan.
The Bulgars also migrated into the Pontic steppes at the time (being roughly 580 AD), starting the Kingdom of Old Bulgaria, which absorbed the collapsing Kutrigur/Utigur Hunnic states and incorporated it into its own confederation.
The Onogur and Sabir Huns would go on to form the Khazars.