You customize corners in the player arena. Go to the skill games section then go to the arena. Then hit the start or back button and you'll get the option to customize set pieces.
Yes you can use them online.
Can you do it with throw ins also? Or is it only free kicks and corners?
the only way to do a long throw i'm pretty sure in the box is to hold X and hope for the best. --- "the only way to ever really know life's lessons is when it smacks you right in the face."- Bane- "In Pieces"