zfs newbie question

[icema]

hi,
i have a new fbsd-8.2 install (dual boot with win7, just desktop general use) on entirely ufs disk, and am not 
sure how to mount a zfs  formatted disk from a previous install, without loosing what is on there. (freebsd-zfs).

in short, the zfs disk was from a previous freebsd install, same version, just needed to wipe/reinstall, which was also
entirely ufs. To try out zfs i used a full separate disk, partiitoned and setup as freebsd-zfs through gpt, then created 
the pool specifying it. i.e   

zpool create foo /dev/ad10.

atm i dont have a pool at all and dont know if i use that command, if it will simple create one and mount ad10, 
retaining the data on it or whether that will just reinitialise, wiping the data in the process.

to avoid surprises, i have been looking around to see if u can create an empty pool (without specifying disk space or w/e),
then try the ‘add’ command instead, to add it there, but i dont see that thats possible so far; and i dont have spare space 
to use in creating a new pool.

so, is it entirely safe to use zpool create foo /dev/ad10 to mount it and retain data, or is there some way to create an empty pool?

thanks in advance