PPM is a very unusual image file format that relatively few desktop programs support. In essence, it's a BMP file but different.
I didn't find any good graphical programs to convert PPMs to images, but if you're savvy with the command line you can use the ppmtompeg program that'll convert the series of PPM screenshots to a MPEG video.
There are also some lua scripts that will do the conversion for you. One might be in the script server but I can't remember.
Well ... Probably it isn't on the script server, the one I remember uses ffmpeg or something to convert it, which would be a separate download. If you found the lua script + download it would work though.