What should I do if there are footprint smudges on a drying polyurethane floor and how can I get it off without adding another coat?

This is a very hard fix. You will have to wait until the polyurethane to dry. Lightly sand the area affected and try to touch up the space. It may or may not look normal. But it's worth trying to fix it before you sand the entire area. There really isn't a lot that can be done until after it dries.