Well, the closest oob to belarus is... well, Russia. The best way to implement Belo. units in game would be to put them in russian OOB.
BTW: These two counteries are in close political / economical alliance (far closer than the rest of CIS) - im not sure of the english translation but its called something like like Union of Belarus & Russia (UBeR - great acronym.
) Polish version of the name is even better: "Zwiazek Bialorusi i Rosji" - "ZBiR", wich literaly means Bandit / Thug
.
I'm not joking, it's official.