This is not an uncommon situation, but loses usefulness as the weight of the trailer increases. I tried it on my 17000lb Suites and the kingpin pulled out of the hitch plate. Also, this will put more stress on the section of the fifth wheel frame where the upper front part is connected to the lower main frame. Some manufacturers void the frame warranty if a gooseneck is used. If buying new, be sure to read the warranty in the manual; if used, then its your call whether to try it or not. The gooseneck hitche adapters themselves are very capable of the weights/stresses, but the frame may not be tough enough.
I would also post this question on a larger forum such as:
MyRVTalk.com or IRV2.com
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired