you can always simply append the data, or substitute it in the data list.
For example, if your data has a 1000 group, you can use SUBST to change it, if it does not have one, use APPEND to append it to the list before you entmod the list.
I must admit though, I am a little confused .... I thought you asked how you could get that information to put into the list, not how do you modify a list already existing.... either way, that should solve your problem, unless of course I mave misunderstood once again.