It's simpler just to put the text field in your portal row. A portal row can be made taller to show a multi-row field.
But there are two different variations of the same idea for what you describe. Both use set field.
a) Set field can copy the contents of a field in the portal row's field to a text field that is global or a field defined in the layout's table.
b) Set field can copy the value of the portal record's match field to a field in the layout's table and an added relationship can then match to the portal record--allowing you to show fields from any or all fields in the record that you clicked in the portal. This requires adding a new Tutorial: What are Table Occurrences? of your portal's table for that added relationship.