Yes its possible,
you may want to add the System.xml dll reference in you solution first to be able to compile.
If that didn't work, may be you should follow another approach like using the System.xml in your c# ( you also need to add the reference )
make all the complicated processing there and make a function that return the result and link that function in your mcml.
here is a nice topic that illustrate what i was talking about
here
Xive.