Late response to this post, but figured it might help.
There is no real elegant way to do this, but I found if you export a 24 FPS .xml from one sequence, and a 23.976 FPS .xml from another project, you can get around the blockade of not being able to mix media that were created at different frame rates.
I opened my .fcpxml with TextEdit:
XML header should have the following:
You may have to either search / replace :
conform-rate srcFrameRate="24"
after the clip name seems to do the job.
I had to do this with some DNG sequences that I had begun to grade in 24 FPS that I wanted to conform in an existing 23.976 sequence. Messy but if you're in a jam it can help.
Todd Sines, director
+ SCALE : https://www.scale.la/
New York, NY + Los Angeles, CA
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connecting the space between + within.