mpe:EditorControl is not really an editor, it is a wrapper control for our EditorProvider system, so it loads a different editor depending on which editor is configured in Site Settings, CKeditor, TinyMCE, or FCKEditor.
You can configure which toolbar buttons are visible (from existing toolbar options supported in the specific editor) as described here:
However, adding a custom button to any editor is a much more challenging thing to accomplish and I have no easy answer for you. You could make a custom editor provider based on the code for an existing one and you could study the documentation of a specific editor to find out how to add a custom button.
Hope it helps,