Does delta-rs support adding an existing parquet file on S3 to a delta lake table?

it certainly does 🙂

deltalake::operations::transaction

?

yeah, you should just need an Add action, the parquet filkes added in the transaction log are URIs as designed by the protocol. This code does probably something like what you're looking to do, and it was written by some idiot, but you might be able to make sense of it https://github.com/buoyant-data/oxbow/tree/main