Editing the toolbar file in /dopusdata\Buttons should be no problem.
A normal button starts with:
<button backcol="none" display="label" textcol="none">
and a menu button with:
<button backcol="none" display="label" textcol="none" type="menu_button">
Exit Opus, create a copy of the .dop file, and append type="menu_button" to any button you want to convert.