I do agree that it is impossible, but...

Last winter I purchased 2 J seats (supercheap I fares) from JFK-BRU. I paid for one ticket entirely with vouchers, and another with an unused ticket and cash. Due to the complexity of the purchase, I could not buy them on the same PNR. A few hours after the tickets were issued I decided I wanted to extend the trip a day. I called up the diamond line, gave the agent both PNRs, and he immediately reissued the tickets on one PNR with the new outbound flight. No change fees.

That flight was cancelled 3 days in a row due to the big Christam nor'easterr, but thats a whole other story.