Since a red dot has no magnification ( 1X ), there's no point in trying to use both the red dot and the iron sights. I've never seen see-through mounts on a pistol, but Weaver-style see-through rings are available.
As an aside, the see-through rings on a rifle are something that sounds like a good idea until you try them. You will find that the overwheming majority of (if not all) experienced shooters condemn their use.