Piano reduction?

I have a four part choral score in Musescore. I need to add a piano reduction of the four parts. I've added the piano staff to my score at the bottom. How do I copy & paste the four individual parts into the piano staff to create a piano reduction? When I try copy and paste the individual parts, each part REPLACES what was there, but I want it to ADD to what was there.

I've searched through the manual but can't find a solution. I hope Musescore allows me to do this -- otherwise I will have to create the piano reduction manually which will be hours and hours of (unnecessary) work!!

Thanks,
Davis E.
Toronto