Someone also suggested disk2vhd
which would probably be good for a once off. Veeam Windows agent for server has CBT for ReFS which would be necessary for repeated backups.
It'd be nice to having something that pulls the image, we're pondering ways to get an offline copy of our long-term (12mo+, 26TB atm) backup copy repository that's updated once a week, both for cryptolocker recovery and in case the whole server dies in a fire for some reason, we don't lose that backup history.
Right now I'm thinking a two stage backup, Veeam agent backup to our old netapp, then a small server that boots up automatically (BIOS/UEFI control?), copies the image over, emails success then shuts down again, but I'm sure this could be simplified.