It would be a lot easier to just fab an extension.
Here's one I did at the last minute for a hunt, this is just a piece of plywood to extend a rifle that was set up for something else. Look close at the butt, someday I'll blend it in a little better.
I have short arms so most of my guns need to be shortened, I save all the cutoffs so they can be added back in, but you can make them from just some hardwood from the lumber yard. If you don't want to cross-grain it you can just laminate a stack.