Didn't think of that.
A workaround could be to use a macro code and assign a shortcut to it.
Dim oValue, nValue As String
oValue = ActiveCell.Value
nValue = InputBox("Altering cell value", , oValue)
ActiveCell.Value = nValue
After putting the code in a standard module, hit Alt+F8 to open the macro window and then click on the option button to assign a shortcut key to it.