I don't know much about FCP (I'm a Premiere guy) but I'd be astonished if it wouldn't handle MPEG-2.
However, in the worst-case scenario, you can use the free tool ffmpeg (apparently downloadable from ffmpegmac.net, but I can't test those builds as I'm on Windows).
A commandline such as this will turn more or less anything into ProRes.
ffmpeg -i "inputfile.abc" -c:v prores_ks -probesize 5000000 -profile:v 3 -vendor ap10 -q:v 11 "outputfile.mov"
- Values of 9-13 are useful for "q:v". This allows adjustment of bitrate. Larger is less.
- Values of 0-3 (and possibly 4, I'm not sure) are valid for "profile". 0 equates to "proxy", 3 equates to "HQ". I'm not sure it supports 4444 yet.
- Audio can be tricky. If you get no audio, add -c:a pcm_s24le after the input file. This will convert anything to uncompressed 24-bit.
- Audio can be tricky again. If you get strange behaviour regarding FCP thinking it has two (or more) mono channels rather than one stereo channel, try adding -filter_complex "[0:a:0][0:a:1]amerge" after the input file.
- Be careful it hasn't adversely affected luminance encoding - check things look to have the right brightness, contrast and saturation. This is usually fine.
ffmpeg's prores encoder is probably faster than Quicktime's under at least some circumstances and I have not had any compatibility problems. This can be a convenient way, depending on your shell scripting capability, to perform bulk transcoding tasks. ffmpeg supports a huge variety of codecs and is generally worth having around.