. . . . one of the video intro and outro templates and customize it and then insert it into my timeline . . . .
Do the reverse - Templates must be added to an existing object on the timeline - then edit it.
AFAIK you cannot save the modified template as a new template, however you can export the modified template as an MVD file which you can add to other projects - File, Manage project, Export movie file and then edit.
Note when you add and MVD file to a project it appears on a separate timeline - you will have to copy and paste the objects to the projects timeline.