You must open world\maps\campaign\imperial_campaign\descr_mercenaries.txt
There you will find different entries like this one:
Code:
pool Armenia ---------------------------------------------------------------------------------------------------------------------------------------------------------------
regions Colchis Media_Atropatene Phanaroia Albania Armenia Pontus Cappadocia Armenia_Minor Armenia_Nova Phrygia Armenia_Maior Pontica ;Troas
unit merc sarmatian cavalry, exp 2 cost 1400 replenish 0.08 - 0.15 max 1 initial 0
unit merc scythian horse archer, exp 2 cost 1500 replenish 0.1 - 0.18 max 1 initial 0
unit AOR thureporoi east, exp 2 cost 850 replenish 0.15 - 0.25 max 3 initial 1
unit merc thorakitai mail, exp 2 cost 700 replenish 0.06 - 0.19 max 2 initial 1
unit merc peltast, exp 2 cost 750 replenish 0.06 - 0.19 max 2 initial 0
just add your unit there, in the "pool" that better suits your unit. As you can see, the regions that the pool incorporates are just below it.
For instance:
Code:
pool Armenia ---------------------------------------------------------------------------------------------------------------------------------------------------------------
regions Colchis Media_Atropatene Phanaroia Albania Armenia Pontus Cappadocia Armenia_Minor Armenia_Nova Phrygia Armenia_Maior Pontica ;Troas
unit merc sarmatian cavalry, exp 2 cost 1400 replenish 0.08 - 0.15 max 1 initial 0
unit merc scythian horse archer, exp 2 cost 1500 replenish 0.1 - 0.18 max 1 initial 0
unit AOR thureporoi east, exp 2 cost 850 replenish 0.15 - 0.25 max 3 initial 1
unit merc thorakitai mail, exp 2 cost 700 replenish 0.06 - 0.19 max 2 initial 1
unit merc bucellarii, exp 2 cost 750 replenish 0.06 - 0.19 max 2 initial 0