How to Extend Backgrounds

SIMPLE BACKGROUND

Most backgrounds are usually just static images.

To extend the duration of a plain image, simply drag it to the right

ANIMATED BACKGROUNDS

Animated backgrounds are made of multiple elements.

To extend them,  double-click on the group to open a view on each track. 

Select all the tracks (Ctrl/Cmd+A), and drag the endpoint to the right.

Click Back to the main timeline to exit the group.