If I book HND-MNL (or some other intra-Asia route, e.g. SGN, HAN, KUL), can I add on ORD-HND at a later date without having to cancel and rebook?

I want to do ORD-HND-MNL in the fall in F and J. There's no F availability on ORD-HND so I'll be waiting on last minute. The problem though is that there is J availability on HND-MNL now, but nothing within the next 14 days which leads me to think that they do not release last minute availability on this route (same for SGN, HAN, KUL). So ORD-HND might become available at T-14, but HND-MNL won't be there at that stage.

I'm trying to think of someway to secure the HND-MNL seats while there are there now and I would prefer to have both flights on the same ticket.