Login to myCourses and go to the course you've chosen to house your merged courses.
Go to the Manage Tab and then select Roster from the list in the Course Management section.
Click on the link for Roster Synchronization found at the top. This will open the Roster Sychronization window, which displays the rosters associated with the course.
Click on the Add Roster link found at the top to add a new roster to the list. This will open the Add Roster Synchronization window.
Check the box to the left of the courses you want included in your merged roster. Notice the two options towards the bottom of the page that are checked by default. See figure to the right.
Once you have confirmed your selections, click OK at the bottom of the page. This will return you to the Roster Synchronization page where any changes should now be reflected in the list.
Click Exit Roster Synchronization at the bottom to view the new, merged roster.