You should read the zfs(8) and zpool(8) man pages, as well as the relevant section in the FreeBSD Handbook. One does not mount a freebsd-zfs partition; ZFS uses pooled storage, and ZFS filesystems are mounted using their own mechanism.
That should be possible. You have a zpool consisting of a single disk (your usb drive). The zpool has a name that was given to it on creation. If you insert your usb stick and run:
# zpool import
The name should show up. This name (eg myfreenas) can be used to mount all the filesystems in the pool in one command, but be carefull. Some of the filesystems on your stick might be overlapping with existing filesystems on your system; so force everything to mount below /mnt: