I was at the same point than you've been 2 years ago. There was a post from hakan discribing how to read / write pdo's. You have to define the TxPDO mapping of the drives via ethercat. I have not found a way to do this inside the ASDA software. Take a look at the attached screenshot. These are my notes but i can't remember it in detail. The Delta manuals are good in general but this part isn't. If requested, I can attache one of my drives config files as well. I'm using absolut encoders though.
Edit: The corresponding chapter on the manual of the B3 drives is: 11.2.2.1 Process data object (PDO)
As far as I understand it, you're defining the "registers of the drive" that get map into registers, that are used to concatenate the ethercat dataframe.
Hope that helps