Hi,
In case of a camcorder that is not specifically mentioned, please note the following text under Learn more in the article iMovie for macOS supported cameras - Apple Support:
"iMovie works with most mini DV tape-based camcorders using DV and HDV formats. Use FireWire port for tape-based devices."
Your information indicates that an iMac with a built-in FireWire 800 (9-pin) port is used. Correct? You may want to check the exact model at https://everymac.com/systems/apple/imac/index-imac.html. The camcorder would have a 4-pin DV (IEEE1394/FireWire) port. If so, a 4-pin to 9-pin FireWire cable could be used.
For newer iMac models without FireWire ports, a Thunderbolt to FireWire adapter, possibly combined with a Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter, can be used. However, that would not be applicable/necessary in this case.
So, it may be well worth carrying out a test with iMovie. An older iMovie manual could perhaps be of some help: https://manuals.info.apple.com/MANUALS/0/MA626/en_US/iMovie_08_Getting_Started.pdf. For details, I suggest that you post a question in iMovie for Mac - Apple Community.
Good luck!