pdeng,
per paragraph UG-115(b), one system of measurement must be used consistently for all documentation and marking of vessels. In your case I would use U.S. customary units and show kg/cm2 in parentheses, check with your AI first. UG-115 and UG-116 are the minimum standard for marking, no restraints on adding additional information, see Fig. Ug-118 general note.
Regards,
RLS