To be honest I havent really tried that before, I do know you can use the mask feature when using pan/crop on a clip and that can blend two clips together so it looks like that. Someone else might be able to help more here 
If its a basic slow motion you could split the video section you want to apply it to and hold control down and on the right edge of the clip drag it to the left, that would cause it to play slow motion. It isnt the best but it works.