Don't know if you are refering to a Machining Centre, but if you are: -
Work Offsets are called using G54-59 or G54.1 P_ on newer Fanuc.
If you are refering to offsetting the Z in relation to compensating for current Tool Length use G43 H_ (Tool Length Offset No.)
G52 Z_ will offset the current work offset also - but you must reset this back to zero in pgm I believe.
These codes are probably the same for lathes, not sure.