Fresh installation on top of a old system in ZFS

[Ross Penner]

I'm hoping to get some guidance on the best way to create a fresh
FreeBSD install using my existing drives partitioned with ZFS.

The old set up is with two drives of different sizes. The smaller one is
mirrored with a partition of equal size on the larger drive to create
one pool which is the root and boot partition. A second pool is made of
the remaining space on the larger drive mounted at /tank/

I booted the USB image of FreeBSD 10 and I couldn't see an obvious way
to use my old partitions as the base for the new system. Any advice that
could be offered would be really appreciated!

Thank you in advance.