You cannot book A-B-C on easyJet as a single transaction. You have to book 2 one way tickets A-B and B-C (or 2 returns A-B-A and C-B-C). So your second flight is not/cannot be a second flight in terms of being put on an earlier flight. All you can do is present yourself, ask nicely and hope. If you start quoting those rules at them I suspect they'll dig their heels in.