Well i would assume might be cheaper to just change the whole ticket to F instead of issuing 2 tickets (1J to SIN and 1F to SYD). Changing whole ticket to F should be possible (as even with F ticket you can fly SFO-SIN in J). But would be good to ask for the price differences.
But what’s definitely not possible is to just pay partially as like the other poster already mentioned there are no half J half F tickets