Welcome aboard David. I agree with Trevor, many conversions were carried out to cars that didn't have an estate in their range back in the 60s and 70s - Ford Zephyr Mk3, Triumph Stag estates, Rover P6 "Estoura" to name just three although to be fair, the Stag estate was based on a 2000/2500 estate with a Stag engine and brakes fitted.
There was also a pair of SD1 Rover estates, one produced in house by grafting on a Volvo 245 back end and the other (if memory serves) by FLM Panelcraft and Ladbroke Avon produced a Series III XJ6 estate :
https://www.aronline.co.uk/cars/jaguar/xj6-xj12/avon-jaguar-estate/
Avon also produced the Avon Acclaim, essentially a Triumph Acclaim with Jaguar-like luxury and also a turbo version.
Many conversions exist as aftermarket, sadly few manufacturers actually build their own estates from the drawing board onwards.