There isn't a script available to do this for you. You'll have to study how Community Server stores the forums and users in its database and then do the same for mojoPortal. Once you understand how the data is stored, you can write SQL scripts to copy the data for you. If you're comfortable with SQL, it isn't too difficult. Migrating the Users is going to be a lot easier than the Forums. Unless the passwords for the users are stored in Plain-Text, you'll have to create new passwords.
I have migrated data from a Drupal site using mySQL to a mojoPortal site using MS-SQL in the past. It wasn't hard, but it was time consuming. In the end I was very happy because it was one less Drupal site in the world. :-)
When you are working on the scripts, you can post specific questions in the Developer Forum and someone may be able to help you.