You can tell a single cell to ignore the activation on the column or cell and only look at it's own Activation property (which defaults to AllowEdit).

So to achieve what you want here, you can do something like this:

        private void ultraGrid1_InitializeLayout(object sender, Infragistics.Win.UltraWinGrid.InitializeLayoutEventArgs e)
        {
            UltraGridLayout layout = e.Layout;
            UltraGridBand band = layout.Bands[0];
            UltraGridOverride ov = layout.Override;

            ov.AllowAddNew = AllowAddNew.TemplateOnTop;
            for (int i = 0; i < band.Columns.Count -1; i++)            
            {
                UltraGridColumn column = band.Columns[i];
                column.CellActivation = Activation.NoEdit;
            }
        }
        private void ultraGrid1_InitializeTemplateAddRow(object sender, InitializeTemplateAddRowEventArgs e)
        {
            this.AdjustRowUpdating(e.TemplateAddRow);
        }

        private void ultraGrid1_AfterRowUpdate(object sender, RowEventArgs e)
        {
            this.AdjustRowUpdating(e.Row);
        }

        private void AdjustRowUpdating(UltraGridRow row)
        {
            bool allowUpdate = row.IsAddRow || row.IsTemplateAddRow;
            foreach (UltraGridCell cell in row.Cells)
            {
                cell.IgnoreRowColActivation = allowUpdate;
            }
        }