You'd have to somehow get the streaming video from YouTube into a file on your hard drive first. Screen capture software might be one option. However, it's not very comfortable to use. There are certain ways to download the file from YouTube, including some websites that will make it pretty easy for you. Try this:
If it downloads as an MP4 file, you can import it directly into current versions of Movie Edit Pro. Unfortunately, a FLV won't work here. It's not supported as import format by MEP. Another conversion process might be necessary. The MPEG-4 stuff you grab off YouTube is pretty highly compressed material, though. Not exactly the most ideal source material for a video editing project, but maybe it will suffice for your needs.