Flash - Extend KmlPlacemark In The Google Earth API?

I'm trying to extend KmlPlacemark in the Google Earth API. Google uses factory methods to create the objects so I can't do something like this:

KmlPlacemark.prototype.foo = fuction(){...}

I tried to simply add methods to each of the objects after they were created

function foo(){...}
placemarkObj.foo = foo;

but it throws

Error: Trying to add unsupported property on NPObject!

I ended up just making a wrapper for it, but I'd still want to know if there's any other way of doing it.