Sorry for warming the corpse...
Again about CD-Extra: I found in the Cue Sheet dialog that it is possible to add a new session. Also I found that *.ISO is a supported track type. Now, if I prepare the data part as ISO image, then create a cue sheet with audio tracks, and add a last data track ... with or without new session ... I'm going to test that later. If you already know it won't work, please tell me!