Yes, you can add as many courses as you want during your free trial period provided you have entered your payment details on the registration page.
To add any course, please login to your account using a web browser and click on "Add New Course or Browse" option in your member area. Then, select a course and click on "Enroll Now" button to schedule the day and time for the classes as per your convenience.
*Note:* On-demand courses doesn't have a live schedule. Hence, these courses will be added instantly to your account as soon as you click on "Enroll Now" and you can start watching the classes recordings.