To me it sounds like UTxO is a very specific implementation of the more general eUTxO protocol. So could eUTxO be (in theory) be used to implement UTxO? Not that one would - but just to understand the distinction between the two.
1 Answer
You can think of regular Bitcoin-like UTXO's as a subset of all possible eUTXOs possible to construct on Cardano. Transactions that simply transfer ADA from one wallet to another (and nothing else) are examples of bitcoin-like UTXO's. eUTXO extends this core functionality by allowing one to attach arbitrary data and logic to each transaction, enabling complex lock-and-key mechanics that go way beyond simple public-private key transfers.
So to answer your question - yes; classic UTXO's are already implemented in Cardano and have been since genesis. UTXO's are to eUTXO's like squares are to rectangles.
Read more about the eUTXO model in the official Cardano Documentation
