It is possible to convert a Movie Project to a Disc Project. With the desired Movie Project open you can either select File > Create Disc Project from Movie while in the Movie Editor or you can click the Disc tab. If there is not a Disc project already open it will bring up the following dialog.
Click Yes to convert the project. The project will then open up in the Disc Editor.
If there is a Disc Project open, you can either close it and then click the Disc tab or just use the File > Create Disc Project from Movie method.
Alternately, you can also add an entire Movie Project or multiple Movie Projects to a single Disc Project. To accomplish this, open Movie Projects in a tab of the Compact Library. This will display your Movie Projects. To add the Movie Projects to the Disc project, just drag them from the Library to the Timeline.