Would it have been feasible to install 'conformal fuel tanks' on the Mercure? The Dassault Mercure was a short-range twin-jet airliner. It was canceled with sales not reaching the breakeven point. Other similar jetliners like the Boeing 737 had a more convenient longer range.

The structural design features of the Mercure precluded increasing the fuel capacity, but there are jet fighters where conformal fuel tanks were added, same as in the Hypersonic experimental X-15.

Are this type of added fuel tanks a feasible option for increasing the Mercure range?
Adding conformal tanks will add drag and weight. Each on its own will impact performance but both together will definitely have a huge impact and I doubt that it would even get airborne.

The reason that jet fighters have these is for ferry flights eg deployment overseas.. and since they usually leave the bombs/missiles/etc behind on these ferry trips there is not a big increase impact in take-off weight.