I do not think you can inherit COM class in .NET you may try interface. i.e. ISketchPoint but it may be not castable from SolidWorks. What you can do is extension methods in C#: http://msdn.microsoft.com/en-us//library/bb383977.aspx.
public static class SketchPointExt
public static void WriteXml(this ISketchPoint point, XmlWriter writer, string nodeElement)
and call it:
SketchPoint skPt = ...;