FreeNas was acquired by IX Systems and as of version 8 is no longer m0n0wall based. IX Systems is actually a great company but I don't like this change. If you need to build a NAS with ZFS try out Nas4Free to keep going with the M0n0wall based FreeNAS that we all loved.
Remove one of the disks from the mdadm RAID1, setup a zpool on that disk, setup the OS, copy the relevant data over, and then simply attach the 2nd drive to the zpool and zfs will automatically create a mirrored vdev for you.
If the existing filesystem can be read by FreeBSD (like ext), you can just copy the data over in a single system but in the worst case, you could just copy the data over from a 2nd system over the network. You'll probably want to do that using tar with archive options, so the only difference between doing it locally vs over the network is whether you pipe the tar create directly back to tar extract, or to a tcp utility like tcputils or netcat in between.
There's no in-place migration, so you either need to copy the data somewhere else, or as the sibling comment suggests, unmirror your drives and accept the risk of having your data only on one drive while you do the copy.
