Crystal Reports :: How To Sort CR Programmatically By The Formula Field

I'm sorting the Crystal Report (2008) programmatically:

FieldDefinition fdSortField = rpt.Database.Tables[0].Fields[ddlSort.SelectedItem.Value];
rpt.DataDefinition.SortFields[0].Field = fdSortField;
if (rbSort.SelectedIndex==0)
{
rpt.DataDefinition.SortFields[0].SortDirection = CrystalDecisions.Shared.SortDirection.AscendingOrder;
}
else
{
rpt.DataDefinition.SortFields[0].SortDirection = CrystalDecisions.Shared.SortDirection.DescendingOrder;
}

(the user selects the sort field from the Dropdown list ddlSort and the sort direction from the Radiobutton list rbSort).

This works fine, the only problem is that this doesn't work for the formula fields, because they are not part of the database. However, since it is possible to set the sort order by the formula field in design time, it should be possible to do this programmatically.