What are the rules on "kitbashing?" (Thunderwolves into bikes)

I'm in love with the wolves in the Thunderwolf cavalry set but I don't really want to run Space Wolves just to use them. My idea was to get a wolf and a generic Space Marine commander kit to make a representation of Kor'sarro Khan on Moondrakken for my own "successor" chapter.

So my question is are there any general/tournament rules "kitbashing" on and using models to represent other things?