I mean... "Technically," you
could port from UDK to XNA. But, that would require you to have access to the source code of UDK. Then, you would have to convert the C++ code of the UE to the C# that XNA uses. And, even then, I say "technically," because from there, I think such a powerful engine would crash XNA!
[Quick EDIT] Although, if you were able to pull that all off, you could make the sweetest Xbox Arcade game
ever! I mean, I doubt the game you'd make would suck if you were good enough to accomplish that feat. Haha.
If life were like a box of chocolates, I'd know what I would get... The one that got dropped on the floor and put back in the box.
Iye nehvur yoose spehl chehk, ahn mie tippyng izz fiyne.