Here's the first thing that came to my mind.
1) Create an item that would catch these monsters, your Pokeball if you will. Make it a projectile item, homing or however you want it to be through the Blizz ABS config.
2) Create a custom event trigger on certain monsters that you'd want to catch. The Custom Trigger event is done by using the "Comment" event command. Write in "Trigger:Item=ID of your Pokeball" without the quotes. Make sure the comment is on the very top of the event page.
3) Create the monster that you want to catch on the Actors tab in the Database. This way leveling up monsters is much easier.
4) When the event is triggered by this item, you can make an "Add Party Member" command that adds that monster from your Actors database into your party.
It's very crude, and I haven't used BABS in a little while so i'm not sure if this would be the best way to go. Also sorry if it's confusing.