ProRes is compressed, but nowhere near as much as the "consumer" compression formats. For example, a 2-hour movie is about 200 GB in ProRes 422 HQ. Aside from some very specific cases, there is no benefit in converting an existing compressed movie into ProRes format.
Compressor can be used standalone and doesn't require Final Cut. It doesn't support as many input formats as, say, HandBrake but it should accept whatever you can play in QuickTime Player. Naturally the files that it produces are guaranteed to work on Apple devices.