The rules seem to cover this without any further clarification needed http://www.dreambuildplay.com/main/rules.aspx

(i) be originally developed, or, to the extent that pre-existing source code is incorporated into the Game, contain only source code that is clearly identified in the Game Design Summary (submitted as part of your Game Description) as coming from another source

vi) not infringe on any third party rights or otherwise violate or be prohibited by applicable law or regulation.

So as long as you note source that is not yours and you don't violate a license you are fine.

Note that the FAQ http://forums.xna.com/37210/ShowThread.aspx#37210 says 'no native code wrapped with a managed layer' - even though i don't see that in the official rules anywhere (though there are a lot of them - maybe i missed it)