Thanks for your message; there is quite some confusion and missing information there!
In Zurich fare network (ZVV), it is not posssible to buy a 1 zone extension ticket, the minumum is two zones, CHF 4.40 full fare. This is by tariff.
On Swiss trains, you cannot buy tickets and you must be in the posession of a valid ticket before boarding the train. If you don’t know how to buy the tickets, kindly seek advice at the staffed SBB Travel Center; there, they also sell tickets and these are the ticket experts.
In the SBB Mobile app, it is not possible to buy tickets for trains with departure time in the past. It is not possible to buy an extension ticket neither.
What the conductor did is absolutely correct and she even applied a lot of goodwill, perhaps because she has noticed you are not familiar with the ticketing system. The conductor sold you a new ticket for the entire route CHF 6.80 plus a CHF 10 service fee.
Had she applied the tariff as she would have been required, you would have paid each CHF 6.80 for the ticket plus each a CHF 75 supplement as travelling with 1 missing zone is considered as “travel with partially valid ticket”.
To conclude, the bottom line is, you both saved CHF 65 thanks to a very lenient conductor.
Have a good travel.