ListMailPRO lets you create up to 10 custom fields for each list. This allows you collect any type of information you like about your users. You can then create custom message codes that will display the user's entered information in any email.
The Custom Fields button can be found in the List Settings page. Simply enter a short description and the field will be considered active. When active, the field will be shown in the user adding & editing pages as well as the signup code generator for the current list.
Note that even if a field is not named, or active, it is still usable in your signup form and message codes. However, the field won't show up when editing a user, or with the signup code generator, unless you give it a name.
Next: Signup Forms
Related: List Settings, Followups, Custom HTML