Can I add a TxManager to a regular, non-transactional store?

Abstract: Can I add a TxManager to a regular, non-transactional store?

Yes, (assuming that you already upgraded to version 3.1)
you can set a new TxManager before opening the store.

TxManager tx = new TxManager();
DataStore ds = new DataStore();

ds.setFileName("TxStore");
ds.setTxManager(tx);
ds.setUserName("user");
ds.open();
ds.close();