As Mwenenzi has said you're going from an A-B via C ticket and wanting to convert it to an A-C (stop)-B-C-A. This will require repricing if the ticket restricts allow. Also said but deserves being repeated, don't skip one leg as all others after it will be canceled.
You say you wish to have a layover. Could you do this layover on the return? If you have no luggage you could get off in MNL on the return and purchase a flight returning to SYD on your own.