I've been looking to do similar. I know you can copy/paste existing cars, edit a config file to designate sirens and such but you'd have to create a whole new model for that light. I know it's possible in Zmodeler3 but that program isn't cheap and I'm currently looking for an alternative. In the mean time, look over this http://www.lcpdfr.com/forums/topic/50909-adding-a-new-emergency-vehicle-is-possible-now/?page=1