Yes you may, just go to any FF counter ask request for a 'Passport' in order for you to work out in FF gym outside Malaysia.

Erm yes i sign FF in Malaysia, just happened that i was asked to go Singapore for a one month business trip, and i only visit the gym twice a week so i did't encounter any problem though. By the way the so called passport was just a temporarily pass for me during Singapore stay.

How much is it for a membership in SG? I guess there are platinum and one club only kind of membership. If you're only about to work out in one gym, can try to use the lower rate membership instead of platinum. For me I personally don't quite recommend to sign in MY and use the pass in SG gym, just in case might get into trouble or anything. Anyway I heard FF offers the most expensive membership among all, maybe you can compare with other gyms first before decide