Adding & removing staves

Is it possible to change the number of staves within a score? In choral writing, it's quite common to split a part into two for a few lines, or into double choir. So a piece might start with four staves, have a section with five (or more), then go back to four again. I can't find a way to add/remove without affecting the whole score.

Thanks,
Rick (mscore built from SVN on Ubuntu Linux 8.04)