I think a dissolve overlaps the frames for the length of the dissolve, so the total time decreases. You can increase the total time to 10 by lengthening the time of one or both frames. In the clip viewer, double-click on the frame and a dialog box will open allowing you to change the duration. In the timeline view, you can just drag the edges of the frame to change duration, although you might have to remove the transition first, then reapply it.
I'm not sure I understand your second question, but generally, if you increase the duration of any clip, it will increase the duration of the whole movie.