(available in EditPad Pro only)

Select Suffix in the Block menu to suffix the selected block with one or more characters.   If you've made a selection that flows along with the text, the characters will be inserted at the end of each line in the selection. If you've made a rectangular selection, the comment characters will be inserted on each line spanned by the selection, after the rightmost column in the selection. The inserted characters will become part of the selection.

See also

Block Menu
Block|Rectangular Selections
Block|Fill Columns