720p at 50 fps to 720p at 25 fps for youtube

Source footage was shot with dslr, in 720p at 50 fps. Motion looks very fluid and smooth. If I convert to 720p at 25 fps, motion looks awful. The only solution I know is to add motion blur. Is there an avisynth filter that works on progressive footage, tracks motion of objects, process frames, and returns new frames with motion blur added ? QTGMC can do this ? Or QTGMC works only with interlaced footage, and 50p must be converted to 25i first ? Because, if something in 720p at 50 fps is uploaded on youtube, it will be converted to 720p at 25 fps, and motion will be awful.