You will need to modify three files to allow recruitment in most cases.
First, as you have found, you need to add unit ownership for your faction or culture in export_descr_unit.txt.
Second, you need to add model and skins in descr_model_battle.txt as follows.
Using the Bactrian Agema Hoplites as an example. Find their entry and add the lines in red.
Code:
type greek_hoplite_bactrian
skeleton fs_s1_hoplite ; hoplite
indiv_range 40
texture britons, xgm/data/models_unit/textures/unit_greek_bactrian_elite_hoplite.tga
texture thrace, xgm/data/models_unit/textures/unit_greek_bactrian_elite_hoplite.tga
texture greek_cities, xgm/data/models_unit/textures/unit_greek_hoplite_syracusan.tga
texture romans_brutii, xgm/data/models_unit/textures/unit_greek_hoplite_syracusan.tga
texture merc, xgm/data/models_unit/textures/unit_greek_bactrian_hoplite.tga
texture slave, xgm/data/models_unit/textures/unit_greek_bactrian_elite_hoplite.tga
model_flexi xgm/data/models_unit/unit_greek_hoplite_bactrian_high.cas, 15
model_flexi xgm/data/models_unit/unit_greek_hoplite_bactrian_med.cas, 30
model_flexi xgm/data/models_unit/unit_greek_hoplite_bactrian_low.cas, 40
model_flexi xgm/data/models_unit/unit_greek_hoplite_bactrian_lowest.cas, max
model_sprite slave, 60.0, xgm/data/sprites/britons_greek_hoplite_elite_sprite.spr
model_sprite merc, 60.0, xgm/data/sprites/britons_greek_hoplite_sprite.spr
model_sprite romans_brutii, 60.0, xgm/data/sprites/greek_cities_greek_hoplite_bactrian_sprite.spr
model_sprite greek_cities, 60.0, xgm/data/sprites/greek_cities_greek_hoplite_bactrian_sprite.spr
model_sprite thrace, 60.0, xgm/data/sprites/britons_greek_hoplite_elite_sprite.spr
model_sprite britons, 60.0, xgm/data/sprites/britons_greek_hoplite_elite_sprite.spr
model_tri 400, 0.5f, 0.5f, 0.5f
Finally, you need to add recruitment lines in export_descr_buildings.txt.
You might do so as follows.
Find the building you wish to recruit the unit from. I've chosen the fourth level of the Barbarian barracks. You need to add recruitment lines to each building level you wish to recruit from.
Then using the type entry from export_descr_unit.txt (that is the one with spaces), add a line like the one in red.
Code:
barracks_four_barbarian requires factions { barbarian, } and building_present_min_level hinterland_culture_barb barb_colony_2
{
capability
{
;infantry
recruit "barb peltast gaul" 0 requires factions { gauls, slavs, }
recruit "barb peltast german" 0 requires factions { germans, }
recruit "thracian peltast" 0 requires factions { thrace, }
recruit "warband sword gaul" 0 requires factions { gauls, slavs, }
recruit "warband axe german" 0 requires factions { germans, }
recruit "warband falx thracian" 0 requires factions { thrace, }
recruit "barb naked fanatics" 0 requires factions { gauls, slavs, } and not marian_reforms
recruit "warband sword heavy gaul" 0 requires factions { gauls, slavs, } and marian_reforms
recruit "barb heavy spearmen gaul" 0 requires factions { gauls, slavs, }
recruit "barb heavy spearmen german" 0 requires factions { germans, }
recruit "barb warguard gaul" 0 requires factions { gauls, slavs, }
recruit "warband dhaxe german" 0 requires factions { germans, }
recruit "thracian thorakitai" 0 requires factions { thrace, }
recruit "warband axe heavy german" 0 requires factions { germans, }
recruit "thracian noble falxmen" 0 requires factions { thrace, }
recruit "barb berserker gaul" 0 requires factions { gauls, slavs, }
recruit "barb berserker german" 0 requires factions { germans, }
recruit "greek bactrian elite hoplites" 0 requires factions { thrace, }
Finally, if you wish, copy the unit cards from UI/unit info/"faction A" and UI/units/"faction A" to the corresponding folder for your faction.
For AoR units, all you need to do is the last step.