You are correct - currently there is no way to extend an existing class or implement an interface coming from a third-party library. The current version of the Android Runtime uses statically generated bindings and the problem is that there are no bindings generated for the external library.
The good news is we have already implemented a new dynamic binding generator and it is successfully integrated in our development version. We are now thinking how to enable latest development bits as npm packages, so stay tuned, the feature is about to be available very soon.
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