I'm not aware of such a tool. I found the best way was to sort my 2012 QSOs by entity, then search (Ctrl + F in my browser) for the entity name in the online entry form (https://p1k.arrl.org/diamond-dxcc/diamond-entry.php
), and enter any that match. There are 231 possible Diamond DXCC entities. Mine took an hour or so to complete. Hit Save frequently.