Yeah it's possible.

What the agent could try to do is to cancel with segments without saving the PNR (this is important), and then rebook the F segment by itself by direct booking it.

Because the segment is cancelled but not saved, the PNR is kind of still holding the F inventory because it's not confirmed cancel yet (so an agent can attempt a change and reprice without losing inventory etc.), so direct booking it back (usually) works.

You'd then be able to tack on the extra segment.

But it's really important that you tell the agent to cancel the segment but do not save the PNR. If the agent cancels and the OAL doesn't return the inventory back, you're SOL.

Have fun.

​​​​​