You can add fields to your portal much like you would add a field to any other part of your layout.
First, enter layout mode and resize your portal row, and/or the objects in it to make room for your new field.
Then drag and drop from your field tool to add a field to the portal. Before selecting the field you want from the specify field dialog box, be careful to select either the table occurrence name specified in "Show Records From" of your portal setup... or a table occurrence that is related to that table occurrence.
Ah gotchya - thanks again!