As Dave already noted, the importusing statement was a reminiscence of Panorama 6. That feature is now part of the importtext statement (see “REARRANGE” option).
During the import, you can already do many things (incl. changing the order of imported fields or calculations), but you don’t have to do so. You can do anything after the import, and you do not need slow loops.
I import a text file with soccer results replacing previous data.
Then my procedure “Auswertung” e.g. calculates (in a separate field) a superdate from the imported fields “Date” and “Time”, and in other fields it calculates points, goal difference and goals, and at last it prepares a typical soccer table.
You can do any calculations with the Field > Morph … menu commands — or procedurally. Write your procedure from scratch or record it while doing the desired actions manually. Call your procedure with the Action menu (or with a keyboard shortcut).