I don't thinnk you can add additional column.

Some reports after generating the report you can go View>Choose Table Columns and set or unset the columns that report has been written with.

If the column you want is not there then your option is to purchase EMC Backup Advisor (EBA) or to gather your data using some kind of script.

EMC intentionally don't give you too much in the way of free reporting in Networker as they would struggle to sell EBA!