Do you mean a FreeBSD image that is literally in raw format?
Besides install ISOs, FreeBSD provides VM images, one of which is a raw image that's supposed to work with iohyve.
How about creating a VM with iohyve where the disk size is an exact match with your raw image and then dd the raw image to the zvol disk0 which iohyve created for your VM.
I got just about that far, but am not sure how to do the dd. I could see the disk0 show up in the GUI, but I could not figure out how to see it from the shell.
Also, can I make the disk image larger and expand later? I don't know how much free space the FreeBSD VM image allows for. My home dir and freenas-build dirs already live in ZFS.
Thanks.
Jeff