I tried this by doing the following steps:
- Imported one 4K clip into FCP project
- Created proxy media and found location of proxy file
- Copied all of the other proxy files to the same location as the first one.
- Imported the rest of the 4k clips and created proxy media.
FCPX still did the same process for creating the proxy files, but in the location only the proxy files I copied across were there. So I guess in that sense it worked, and would save some memory space.
I was more hoping for a work-around that didn't involve FCPX doing any processing other than just linking to the existing proxy files.