Can I fully populate a map with NPCs?

Hi

So I've just finished fully fog-of-warring my map of Brandeskar prison (Way of the Wicked) and now I want to plop down all my NPCs to make the game run fast once we start playing.

Problem is, if I roll initiative in game, ALL the NPCs/monsters in EVERY room in the entire map roll as well. How can I "turn off" the initiative of an creature so they don't participate in a combat until I "turn them back on"?

Thanks

 

Thanks!