You can replenish mercenaries in any settlement where there are same type of mercenaries currently available (i.e. you are using up the recruitment pool).
So if there are merc eles still available in Arbia then yes you can retrain them there. If not then no. In that case you have to find a region of yours where you have merc eles available to recruit (and put the merc eles in the recruitment que there).