If there is, you won't be able to add them without overwriting what you're importing now.
I'm not looking to overwrite this Actor data in future, simply trying to automatically add what I've got now as a one-off.
Neither. It works as described.
Well I can promise you that I wouldn't be wasting my time (and yours) if it worked flawlessly for me. I'm curious that you did your experiment with CSV data, when my post was about Excel problems and the current manual says:
The best tool for doing that is a spreadsheet. If you have Excel, use that and the Excel import plugin.
I did briefly try CSV, amid hours of variations, but I don't think I even saw the 'merge options' dialogue appear so reverted to Excel. I stopped trying to use 'Path' early on and changed to ID followed by Actors as the only two columns in the Excel file, both with & without headers in row 1.
I'm using Excel 2003, and saving in the default .xls format in the absence of any other instructions in the manual. It did cross my mind that Excel versions might matter, but it was one variable I didn't try. Is it likely to matter?