These Macs are reportedly shipping with macOS 27 Golden Gate. You'd have two incompatible constraints: Macs don't like to run a version older than what they shipped with, and Golden Gate doesn't support the Intel iMac.
You might be able to boot from the internal drive, install/upgrade the external drive, then boot from it.
You might be able to boot from the internal drive, install/upgrade the external drive, then boot from it.