Just a list of locations from the portal table could be produced simply from a value list based on that table. Just a list of locations from the original field could be produced in the same manner. Both lists would be sorted in ascending order. But that would not produce a combined list sorted in alphabetical order.
To get a combined list, you'd need a set up (custom function or script ) that loops through both lists of locations and generates a merged list in ascending order.
I think I'll set up a new table. I'll make the location field have unique values and then I'll import the locations from the portal table. Then I'll import and tick the add new records for the rest of the locations in the other location field.