With out looking at the adaptor I can only say that charging the pencil may be possible through an adaptor, but it is highly unlikely you can pair the old pencil with the new iPad Pros this way. The new Pros do not pair through the port USB-C port. They pair thorugh the wireless connection on the edge. They would not be expecting a pairing request through the port.
Vice-versa would also be true. You might find a wireless charger that will charge new pencil, but you would be unable to pair it with an old iPad Pro, because the pencile does not have a Lightning connector.
Pairing the pencils appears to be te biggest obsticle, not the charging.
It’s also possible that Apple has tweaked the underling technology in the pencil and iPad, so that they are not compatible. We don’t know.