Sorry to disappoint you.
But no where it is written that PRO version support Group Messaging. All features are linked to docs so that nobody misunderstood anything.
Answer your question:
as you are happy to amend the code, assume you have php knowledge.
1. in class-fep-directory.php line around 71, add another column (div) for checkbox name = fep_mr_to and pass user id as value. wrap all column inside a form and form action will be your new message url and form method will be POST. this will fill “To” field with selected users.
2. For any role you can send announcement by going to Dashboard > Front End PM > All Announcements > New Announcement. Right sidebar there are all roles, just tick which role you want to send this announcement. Remember users cannot reply to announcement.