Any way to 'migrate' a Time Machine external for use with CCC?
No, there is no way. Time Machine uses Symbolic links to previous backups when it hasn't changed. You could use some form of ls -H combination with xargs to copy the latest backup some what safely, however, some times those symlinks are broken. 

Furthermore, CCC won't accept that, because it also need to record which files it has copied, so in future it knows which are changed etc. You would be better off using a removing TM completely, format the disk, or create a partition for CCC. Though no point in keeping two backups on one external HDD, as both will be lost if it's destroyed.

CCC is very fast, it's not as slow as TM, so it shouldn't take long, as long as HDD has USB 3.0 (true, if inside of plug USB is blue). I was in the same place as you, where I didn't want to give up my previous back ups of TM. So ended using both, Time Machine to store all data, and CCC to create a bootable external copy of my disk. CCC only backs up the essentials, while TM does everything. When my PC's SDD is no longer functioning, I can boot up from CCC, and use TM to retrieve required data on the bootable.