Change the sequence settings to 1920x1080, FCP X does the rest for you.
Export a 1920x 1080 from that sequence and a 5K from the 5K sequence.
It's really that simple.
I would suggest the following (and I do it all the time)
- IMport r3d files.
- Create proxy media
- set editing preferences to PROXY (low resolution, ETC
- When you are completely done, set the preferences to HIGH QUALITY MEDIA and HIGH PLAYBACK and FCP will read and render from the R3D so you will get the very best quality on render out.