I own double triggered double guns of same make, one a straight grip, one a semipistol grip. I prefer the feel of the semipistol grip.

Is it feasible to have a stockmaker or cabinet maker alter the straight grip to semipistol grip configuration by adding [matching] wood, then shaping it? Feasible is one thing. Degree of difficulty is another. Would I be getting in over my financial head?

The cost of replacing butt stock, and matching it to foreend, would be very expensive. Same with replacing butt stock and foreend.

Feasible..yes....would it look good, I don't think so. If you look at some old pics of Trapdoor Springfield target rifles, there was a piece of wood that was screwed on the bottom of the stock that gave the stock a pistol grip of sorts. The one I saw could be easily removed as the lower trigger guard tang was drilled and tapped for a screw. When not in use, the owner put a plug screw in it. That might be an option for you. I would really advise against doing anything permanent to the stock because, as you say, it would be very expensive to fix.

i did that once - once - never again - turned out worse than voting for obama

I've seen some (on old Sharps rifles) that were made of ebony with foliate shapes similar to the Euro horn grips found on older drillings and combination guns. Can be VERY attractive in either ebony or horn but probably won't look or feel much like your other one because of the necessary curve.

I did one in Rosewood years ago for one of Ben Shelor the Infamous Engraver's low walls but didn't take photos. Am doing one now in ebony for one of mine, will show photos when I get enough tuits to finish it.
Regards, Joe