unfortunately, I don't know of anyway to globally include all of the methods within a dll like user32.dll.
What you have to do is import the individual functions such as
[DllImport("User32.dll")]
public static extern int MessageBox(int h, string m, string c, int type);
So, you would need to know all of the functions that you would use from user32.dll or windowsx.dll and include them all individually.