1. You can create a DPX straight out of FCP 7 with third party tools.
2. Create an XML of the sequence. open in Redcine X and export DPX files from there.
3. Create prores 4444 from FCP 7. open in FCP X, EXPORT DPX IMAGE SEQUENCE from FCP X
Number 2 is the best quality.
3 and 1 are secondary choices and will not be as high quality as going from source material.