I'm switching to Ubuntu server from FreeNAS and I don't have a hard drive to back the data up to. Is is possible to import it into Ubuntu even though the data pool is split between two different HDD?
As Bruno Pereira has said, the zfs version number from FreeNAS needs to be equal or smaller than the zfs version number ubuntu supports. After zfs v28, this switched over to the need for support of the enabled zfs options.
For 16.04, the versions of zfs and FreeNAS 9.10 or even FreeNAS 10 are equal and a FreeNAS pool can be converted. I just converted a FreeNAS 10 trial installation myself for use under 16.04.
The version of the ZFS on Linux is not compatible with the version you used for your FreeNAS system, you will not be able to import it using ZFS on Ubuntu.
The other way around is possible (importing older versions of ZFS using FreeNAS).
There is no downgrade option for ZFS.
The issues is not the way ZFS is set (pools over different disks, raid setup etc), it has to do with options - mainly datasets - that are present in the FreeBSD release of ZFS, that are not present in the ZFS on Linux release.