Personally I'd just use the existing fields.. e.g. if you want them to hit harder give them higher combat stats
An easy way to do this is to create a separate table with all the NPC_ID's for 'raid mobs'. Then you can run a query with an = or join on the NPC_ID.. For example,
Code:
update npc_types set npc_types.atk = 2 billion, npc_types.accuracy = 3 billion where npc_types.id = raidmobs.id
would give 2 billion attack and 3 billion accuracy to mobs whose ID you've put in your raidmobs table.
I use something similar to balance non-boss mobs by their level.