I suspect the cost would far exceed the value of the finished product. I am not a big fan of trying to make a shotgun into a rifle, especially something made in a 3rd world country. Probably the cheapest way would be to find another huglu and have the barrels Mono-blocked, regulated and fitted to the action. This would also give you the for end iron you need although finding a forearm wood to match and fit the rifle barrels would still need to be worked out.

Then you would also have a balance problem, if balanced as a shotgun it will be real muzzle heavy as a rifle. The stock on a shotgun is also different than on a rifle.

If you ordered a Chapuis, Verney-Carron, Merkel, or something similar you can figure as a good estimate that the 2nd set of barrels will add about 40% to the cost of the rifle.