I did this without creating an extra column in the list. I added a new single-line text control and connected it to the same column. I kept it in disabled mode (this is necessary since the form won't work if more than one control tries to update the same column). Also set its 'visible' property to the expression "Is Edit Mode", and set the visible property of the original combo box control to the expression "Not(Is Edit Mode)" - I did not want both fileds to be visible at the same time.