In aluminum, it's been done with the various Pregnant Guppy/Super Guppy conversions of the B377, as well as the one-off Canadair CL-44D4. There are more that someone will mention...
I'm sure it can be done with composite, the same way plywood-skinned aircraft structures are repaired or modified: the joint is "feathered" so there's a large surface area for bonding. Will it be economically feasable? The jury is out.