I think it’s possible to do this using a somewhat convoluted procedure. I think you must add and then sync the transcoded material to your existing multicam clip, then translate the clip timecode of the existing clip in the timeline to the equivalent timeline timecode of the full-res clip in the multicam.
Note you can open an existing multicam clip, at the left of the timeline click the down arrow and pick “add angle”. Then drag/drop the new angle (which in this case is the low-res transcoded clip), then select that, click the down arrow at left of that lane and pick “Sync selection to monitoring angle”.
After that the procedure is something like this:
1. Use skimmer to get starting timecode of parent clip of low-res clip in timeline.
2. Observe clip length in yellow numbers below viewer
3. Open multicam (MC) and skim to timecode for clip obtained in step #1. Note: it won’t show the clip timecode if that’s the monitoring angle, so select a different monitoring angle before skimming the clip.
4. Place marker; playhead jumps there
5. Move skimmer up so timecode from start of MC container clip displays (vs clip timecode).
6. Go back to timeline
7. In Browser, skim in MC clip to timecode obtained from step #5.
8. Mark range with length obtained from step #2. Expand view as needed using CMD + and CMD -. Use skimmer for rough position then JKL and IO keys to mark the range.
9. Select parent clip in timeline
10. Do OPT+R for replace edit of timeline clip with marked range of MC clip which contains other angles and good audio.