It would vary from kit to kit. Depending on the shape and configuration of the doors, it might be possible to create hinge mortises in the body and doors with a very fine-tipped Dremel grinding bit... it might be easier to scratchbuild styrene door posts by layering thinner pieces together but leaving gaps to create the mortises... it might be possible to cut away the jamb and replace with sheet styrene pieces cut to shape, etc... it all depends on the specific kit, how it's engineered, and how/where the doors are hinged.
On this particular kit it's relatively easy to hinge the doors, because the hinge posts are separate pieces (as they also were with the stock body), and because the doors are straight on the hinge side. Doors with curved openings on the hinge side would be trickier to do.