I'd never researched this myself and have never used a product like it, so take my opinion with a grain of salt.
The Internets say that fifth wheel pin boxes aren't setup/strong enough for gooseneck hookups. Cracks, breaks and general badness can be expected. That said, like above - you'll also read about people who have done it successfully for quite a while.
You may want to contact your fiver manufacturer directly regarding warranty. Again, the Internet says you'll void your frame warranty if you use one.
I use the B&W Turnover Ball gooseneck with the Companion Fifth Wheel hitch. It allows me, with relatively little work, to switch between gooseneck and fifth wheel if I so choose. I don't have a reason, but the option is there.