People buy and use them.....gooseneck adapters
Aftermarket has bunches for sale
My only problem is in the structural design of a 5th wheel vs a gooseneck trailer
5vr.......long narrow outside frame strut with short narrow cross beams to attach pin box,supported by narrow, wide placed uprights and no bracing
Gooseneck trailer......wide, heavy upright support, thick heavy frame stuts that converge at point of gooseneck with heavy bracing
Structurally....5vr just isn't designed for gooseneck use
BUT folks buy them & use them.........it is their trailer to do with as they see fit. Even if they are wrong