It sounds like nobody actually understood what you want.

You have a non-ZFS boot drive, and a big ZFS pool, and you want to save an image of the boot drive to the pool, as a backup for the boot drive.

I guess you don’t want to image the drive while booted off it, because that could produce an image that isn’t fully self-consistent. So then the problem is getting at the pool from something other than the system you have.

I think what you need to do is find something else you can boot that supports ZFS. I think the Ubuntu live images will do it. If not, you can try something like re-installing the setup you have, but onto a USB drive.

Then you have to boot to that and zfs import your pool. ZFS is pretty smart so it should just auto-detect the pool structure and where it wants to be mounted, and you can mount it. Don’t do a ZFS feature upgrade on the pool though, or the other system might not understand it. It’s also possible your live kernel might not have a new enough ZFS to understand the features your pool uses, and you might need to find a newer one.

Then once the pool is mounted you should be able to dd your boot drive block device to a file on the pool.

If you can’t get this to work, you can try using a non-ZFS-speaking live Linux and dding your image to somewhere on the network big enough to hold it, which you may or may not have, and then booting the system and copying back from there to the pool.