Changing number of staves

Hello there!

I tend to copy up a friend's handwritten compositions on Musescore - it's great! I love it and I've just downloaded MuseScore 2!

However, the latest piano piece he's handed me changes from 2 staves to 3 staves mid way through the piece - and I'm struggling with it. Is there any way I can do this?

Thanks so much! :)