[lmi] "Ctrl-+" keyboard accelerator

[Greg Chicares]

On lmi's "Census" menu:
  Add cell   Ctrl-+
...exactly what does "Ctrl-+" mean? There are three reasonable interpretations:
  (1) holding down Ctrl, press the key labelled "+" over "="
  (2) holding down Ctrl and Shift, press the key labelled "+" over "="
        because Shift normally chooses between "=" and "+"
  (3) holding down Ctr, press the "gray" numeric-keypad "+" key
Today's actual lmi behavior:
  (1) performs "Add cell", but (2) and (3) don't
Desired behavior:
  (1), (2), and (3) all perform "Add cell"
[Cf. firefox, where all three do the same thing.]

Vadim--how can this be accomplished in lmi?