Sure you can refinish individual rooms. It just might take some work to get it to match the other floors perfectly.
If the rooms are set at different levels then this is a piece of cake. If the flooring is contiguous between them and there are no "threshold" breaks of grain direction then this becomes more of a problem.
Lap lines in the finish and the sanding of each room makes the transitions difficult if not impossible to hide. It is common practice to do all continuous areas at the same time.