You'd have to modify the core code to do this. Take a look at function qa_load_editor(...) in qa-app-format.php. You can add the following to the start of the function to make editors and above always use a rich text editor:
if (qa_get_logged_in_level()>=QA_USER_LEVEL_EDITOR)
$editorname='WYSIWYG Editor';
I've not tested this, so sorry for any mistakes!