Hi Ren,
basically you can use MAC addresses. Just create IP reservations in the DHCP server options (as long as you are using the SSG as DHCP server of course),create a group for those addresses and apply it to the appropriate policy.
If your SSG is not running the DHCP server you might want to check your DHCP if it's capable of IP reservations as it's quite a common feature.
Cheers,
flo