Serialize undisclosed c#-object to XML as XmlRoot

Question

I am trying to serialize an gbXML format from data that i extracted from a CAD model in the architecture software Revit. For every element in the CAD-model, i extracted the geometry as a polygon-loop. Thereby, i used the classes of the API of Revit, which are of course undisclosed to me. So now i wonder how i may add the desired XmlElements as children of classes i don't have access to. This is the schema i need to generate:

<PlanarGeometry>
   <PolyLoop>
      <CartesianPoint>
         <Coordinate>26.20337</Coordinate>
         <Coordinate>15.67507</Coordinate>
         <Coordinate>519</Coordinate>
      </CartesianPoint>

And this is the class i have access to, whereby the CurveLoop is a class from the undisclosed Revit API i cannot put any tag into.

[Serializable]
[XmlRoot("PlanarGeometry")]
public class PlanarGeometry
{
    public CurveLoop OuterLoop { get; set; }
}

I did not find any method of the writer.writeXML being able to write non-primitive types. Might there be any solution with extension methods? I would highly appreciate any help! Thank you!

Answer 1

thanks for your help! Answer 1 would work, but be a lot of work... Answer 2 does not work i think, because i need to add two levels of XmlRoot before inserting my new xmlElements - and as far as i have seen, writer.writeXML is only able to write primitives, no XmlRoot!?

The answer i found was to replace the undisclosed classes through classes defined by myself, e.g. CurveLoop through an own class PolyLoop with a List. Thanks!