Yes - - - go to >Options >Advanced Options> Default Services and set it there.
Be aware tho that even if you have your fav email set as the default, if you use a different email address to send to a certain contact the BlackBerry will continue to use that other email address anytime you send to that contact again in the future unless you change it again by sending from another email - - it really is kind of cool feature because I have one contact that I email a lot and their work email server would only accept a certain email that I send from so anytime I send to this contact the BlackBerry knows to use the correct email rather than the default.