Hey,
I would suggest making a copy of the Listbuilder screen and looking at the actions on the Process All button. In the action list (at around action 23) you will see the Character Function being used and specifically the merge fields function. What this does is it takes database terms in the email template (your merge fields) and converts it for the table and field it was meant for. That function allows you to set the table that needs to be referenced for the merge field.
You can find out more about the merge fields on our Help Center here: Merge Fields
See if that puts you on the right path, if you need further explaination let me know.
- Mortaza