Yes, and no.
There are two methods to using ZFS:
* everything on ZFS with UFS /boot, or
* everything but / on ZFS
The second option is easier, and allows you to boot to single-user mode and use /rescue/* when needed.
A nice and easy setup if your system can boot off USB flash drives is to copy / (not /usr, /var, /tmp, or /home) onto the flash drive, use fdisk -B /dev/da0 (or whichever device it shows as) to make it bootable, then boot off it into single-user mode.
At that point, you can configure the ZFS pool and create filesystems for /usr, /var, /home, and so on and set their mountpoints. Then copy (find|cpio, or dump|restore) the data from the UFS harddrive to the ZFS filesystems.
Reboot using the ZFS harddrive to make sure everything still works correctly. Finally, add the UFS harddrive to the ZFS pool. Voila! A system that boots of UFS and stores everything volatile on ZFS.
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