If you want to do a manual ZFS install, just search for the "ZFS madness" thread on here. There's been a few changes in recent versions (such as vfs.root.mountfrom no longer being needed, but the instructions in that thread still work well).
The steps to do a manual install are pretty straight forward. You're really just creating a pool and extracting FreeBSD base/kernel to it. There's just a few important steps like adding the correct bootcode, putting the zpool cache in the right place, setting the zpool bootfs property, etc.
