peter wrote:

I hold a paper travelcard for zones 1-4 (issued at a national rail
station), and occasionally need to travel to zone 5. Until last
December I always paid the difference in cash before the journey, but
the penal hike of cash fares meant that I have now bought a pay as you
go oyster. At the time of purchase (at Victoria TfL enquiry office) I
asked how I should use it for a such a zone extension - if I touched in
at the start of the journey I would presumably be charged for the Z1-4
section for which I already hold a travelcard. Yet once on the train
there is no way to validate the ticket on passing into Z5. The reply
was to tell staff at my destination station who would then deduct the
correct amount from the card.

Probably inaccurate advice. One side of the leaflet with my student oyster
tells me to break my journey to touch on the pad for some extensions
(although the other side says it's an automatic extension).

Well, yesterday I tried this for the first time, travelling to
Cockfosters. Staff there absolutely refused to deduct the correct
amount, saying I should put the whole journey on the oyster card. I
pointed out (politely) that I was following TfL advice, but that made
no difference.

Typical case of not singing from the same hymn sheet. Ideally one should
always get an official TfL leaflet carrying the same advice as given by
staff but in practice this doesn't work.

After about ten minutes arguing they let me through the
barrier (so I got a free ride), but this is clearly unsatisfactory.

How should I temporarily extend the range of a paper travelcard using
PAYG oyster?

The only thing I can think of is to break your journey where the
travelcard's validity runs out and touch in, or alternatively get Oystercard
next time. Are National Rail able to issue future season tickets on Oyster?