Is it possible to "Add tag" to the list of functions that you can assign a short cut to?

If i would like to tag 15 min regardless of the time of day ,or such tag overlapping my last tag, i can not use the shortcut to "tag last untagged time". This is however possible if i chosse "Add tag" from the program window". I would like this function to be more accecible, as i quite often need to include some billing time without fixing it to a specific time on the timeline .