It depends if the Guitar Hero 3 songs were released as DLC on GH: World Tour. There is no way to directly import songs from Guitar Hero 3 onto World Tour.
no
NO
Yes, there are downloadable songs for guitar hero world tour.
Guitar Hero World tour "Beat It"
You need to import them
no.. not at this time
no you can not ONLY in guitar hero WORLD TOUR.
You can add your own custom songs to Guitar Hero World Tour. You cannot create custom songs through the game however.
No
No.
impossible, unless you use hacks on the computer