From what I've heard, new race entries on the character creation screen are only possible with engine extensions. There are many triggers in the base game and in mods which depend on the character's race. Thus, if some NPC likes to romance humans, they'll romance any human subraces the same way. (Icewind Dale II treats Aasimar and Tieflings as human subraces.)
Officially, what is the plural for "Aasimar?" I thought "Aasimar" meant singular and plural, like sheep or moose.