Look in the EDB for the conditions for recruitment of the unit, note down the hidden_resource name.
Code:
recruit_pool "Longbowmen" 1 0.1 2 0 requires factions { england, } and event_counter welsh_longbow 1 and hidden_resource aor_england or hidden_resource aor_wales
Go into descr_regions and add the name for that hidden resource to the other aor entry.
Code:
Caen_Province
Caen
france
French_Rebels
170 170 11
atlantic, explorers_guild, woodsmens_guild, aor_france, aor_wales
5
2
religions { catholic 85 orthodox 0 islam 0 pagan 13 heretic 2 }
There are two hidden resources (england and wales) to allow recruiting of this unit, but the condition is that either is valid. So you will only need one of them. If you use the england part you will make a lot other english units recruitable in that region as well. Using wales restricts this much more. (2 or 3 welsh units)