Why not, it worked sooo well for the Romans in 378 A.D.
Oh wait...
Actually, it worked for the Romans for about 300 years. The Romans enlisted subject peoples as specialists, notably cavalrymen from Gaul,Batavians [who were instrumental in conquering Britain] as auxilliaries. After serving 25 years, they received Roman citizenship.
The problem later was that the Romans enlisted foederatii directly into the Legions, because Romans were no longer signing up to serve in the necessary numbers. The Romans also stifled assimilation, and reneged on bargains with the tribes they enlisted [usually for land].