I probably misspoke when I generalized all MMA fighters or kickboxers as being unable to transition to straight boxing.
I dont know about that... Of those you mentioned, I am absolutely sure David Haye was never a kickboxer of any note, though he did do a little karate as a kid. Povetkin, again, may have done some kickboxing in his youth, but to say he 'transitioned' from that to boxing is again inaccurate; he was boxing from a very young age. Yes Vitali was an international kickboxer, but he also boxed from a young age I think (though not sure on this one).
Matt Skelton was never an MMA fighter; he had one fight I believe which he lost in embarrassing fashion, though he did have some decent performances in K-1. As for his boxing career - I'd recommend giving it a miss, he makes Jawny look like Pac :yep
Povetkin was a competitive kickboxer i believe so was vitali of course and Troy Dorsey. Everyone who has fought kickboxing or full contact karate has probably trained in boxing so there is a cross over. Nigel Benn is one as well.
I looked it up Povetkin was an amatuer KB champion in 1997 and 1n 2000 he was a professional i believe European Champ