Just to correct an error - it is in fact possible to recruit a unit tagged as "mercenary_unit." The only differences between this type of unit and a "normal" unit are that it can be placed in the mercenary pool (descr_mercenaries.txt) and it will use the mercenary texture in descr_model_battle.txt, no matter which faction recruits it.

Other than that it will be recruited just as any other unit will be. You must add the faction to the ownership line of the unit in export_descr_unit.txt, add a recruitment line (or multiple lines) in export_descr_building.txt, add the hidden resource to the hidden resource list at the begging of export_descr_building.txt, add it to the unit's recruitment line as you have done, and add it to the region's resource list in descr_regions.txt. Then, delete map.rwm and voila! It should work fine.