There is no clear document states ScreenOS could perform nat66 (at least Juniper does not use the term "nat66".) However, if one could follows ScreenOS release notes carefully, it became consequences.
set policy id 1 from "Trust" to "Untrust" \
"Any-IPv6" "Any-IPv6" "ANY" nat src permit
set policy id 1
Where obviously, key word "nat" does the trick!