I have to ad up in individually in cells innumerable amounts, unfortunately I
have to jump from one cell to the other, put the cursor at the end of the
already entered formula (something like this: 67+45+2+89+7897+124457+5+2154)
press "+" and the additional amount.

Is there a Private Sub that would add the plus-sign whenever I select a cell
and press a certain simple key?