I would look at LoadMerc around line 5021 in merc.cpp.
Code:
npc_type->gender = gender;
npc_type->loottable_id = 0; // Loottable has to be 0, otherwise we'll be leavin' some corpses!
npc_type->npc_id = 0; //NPC ID has to be 0, otherwise db gets all confuzzled.
npc_type->race = merc_template->RaceID;
npc_type->class_ = merc_template->ClassID;
npc_type->maxlevel = 0; //We should hard-set this to override scalerate's functionality in the NPC class when it is constructed.
Merc* merc = new Merc(npc_type, c->GetX(), c->GetY(), c->GetZ(), 0);
merc->SetMercData( merc_template->MercTemplateID );
database.LoadMercEquipment(merc);
merc->UpdateMercStats(c);
Or, just add it to the constructor.