To insert column of symbol or gap in the Chromatogram Editor select the nucleotide and press Shift+I or Edit–>Insert character/gap context menu for switching the edit mode on. 

 While the selection is blinking, press a keyboard character that you want to insert. 

 After that the edit mode is switched off. For insert another symbol press Shift+I again.

 You always can cancel the changes pressing Ctrl+Z.