Why not just use the Series Tag in Edit Metadata? I just recently discovered it myself & it's beauty! You name the series & you set the starting number. The rest just follow through. I did this for a couple of series - Harry Potter & Blossom Street. Name the series and then tab over to the series number, and enter 1.
Select the first book in the series, Edit Metadata, name the series, tab to number and enter 1. Select the next book, Edit Metadata, select the series name from the dropdown, tab to the number and it automatically enters the next number - 2. Continue for the books in the series and you're done.
Granted though it will make a folder for that series and I wanted that but if you don't, then I don't know how to not make it do that. No need for messing with the plug ins to get the series order though.
HTH