Slightly illogically, you can add more than one series, but only one volume number. I'm adding the Avengers: Time Runs Out collections and they've got comics from Avengers (vol 5.) and New Avengers (vol. 3). But, while I can add both series, I can only put the volume number for Avengers.
Yes, I'd also commented on this. I think the series, volume and issue fields are linked and, if you add a new series, you'd get another volume and issue field.