Fastest way to move XML document into and back out of CLR function

In SQL Server 2005 and Visual Studio 2005, what is the fastest way to pass
an xml data type variable into a C# CLR function and retrieve it back out of
the C# CLR function when the C# CLR function is done modifying it?

I tried this but get deployment error becuase SQLServer2005/CLR dotn support
System.Xml.XmlDocument:

public partial class UserDefinedFunctions
{
[Microsoft.SqlServer.Server.SqlFunction]
public static int InsertSomeNode(System.Xml.XmlDocument pDoc)
{
//
return 1;
}
};

thorws error:
Error 1 Column, parameter, or variable #1: Cannot find data type
XmlDocument. InsertSomeNode