It has been quite some time since I used Monarch 8 so I cannot be sure of it. I am not aware of any way of a user adding any new buttons to the toobar either. I am running Monarch V10.5 Pro and when I click on File on the menu bar in the drop down box on the right side of it where it says Close Model it lists the shortcut keys of Ctrl+D. I think that would be true for the older Monarch versions as well. That may solve your problem.
This might be a feature Monarch may want to add to future versions of Monarch.