You cannot add pages, unfortunately. Annoyingly the U.S. just discontinued this service at the end of 2015, so U.S. passport holders now have the same problem.
One option is to get on a plane to the U.K. and head to the Passport Customer Service Centre (with a prearranged appointment, which is required). You can get
a new passport in 4 hours with "Premium service," and the fee isn't that much more. The plane ticket to the U.K. costs something, of course. Make sure you get a "jumbo" (48 page) passport. If you can manage to make a relatively quick stop in the U.K. as part of your next trip, great, that should solve the problem. Best of course if you're headed vaguely in that direction anyway on your next trip. London Heathrow is not exactly the most common connection airport when you're flying from Singapore Changi to Jakarta.
The U.K. can issue a second passport if you have a justification to have one -- and you very well might if you're filling up pages rapidly and traveling frequently for business. So after you get this immediate issue behind you, see if you can apply for a second passport. The second passport has a different passport number and is simultaneously valid -- or, more precisely, has its own validity period -- along with your first passport. Then you can manage across passports. You would always use the same passport with the same number to enter/leave Singapore (and any other country) -- try to make sure your entries and exits match up everywhere -- but the second passport would be most handy for those times when you're getting visas and the visa country holds your passport for a while. So you fill your second passport with visas (and entry/exit stamps for those visas) and your first passport with entry/exit stamps for visa waiver countries, basically -- at least, that's one approach. When one of them gets close to full, you renew it but still have the other. (Notify Singapore if you're switching your Singapore entry/exit to another passport number.)