Inserting a new note into an existing chord (without overwriting)

Say you've inputted a chord using the MIDI keyboard, and you later want to add one more note to that chord. Clicking on a note in the chord and going into note entry mode, any key on the keyboard will now overwrite the entire chord. It would be good to be able to toggle "overwrite mode" (for example by using Ctrl+Insert), although I realise this may conflict with MS's inner workings. Still, if possible, this would be great.