The cause of the higher humidity in the addition is the mass of concrete and brick. Acutally they are more prone to condense the moisture that is in all of your house. I would not plug the holes in the brick because masonry just draws moisture out of the air and , if moisture gets on the back of the brick, it will need an exit. The crack in the floor could be filled though, it may be allowing moisture from the earth below to infiltrate. I would enlarge the fine crack first so it will hold some good caulk. When I have a cold joint like that (that doesn't need expansion material) I rake out a half inch deep with a 16d nail before the concrete sets. That gives me a good opening for caulking. Since your concrete is set, the opening could be made with a 3" side-grinder. Just protect your eyes, face and breathing from the dust, it is carcogenic.