Not if the Dun is a true dun there would be no cream gene to give you a buckskin. Now the Dun COULD be a dunskin in which case you could get a Buckskin dun skin and so on but if he is a true dun then no chance of a Buckskin.
I'm good with cream genes but I don't really know much about dun. Does it act similar to the cream gene? Lightening the coat significantly? Or just adding the primitive markings - Leg/wither barring, countershading, dorsal stripe? You can have dun over black base + agouti + cream which would give you a dunskin, yeah? How would dun present over a true black with no agouti?
Oops missed the part where you mentioned a dunskin.
What would you call a black base + no agouti + 1 cream gene + dun? So a smoky black with dun? Or does dun only express on agouti... Hmmm. Off to google :]