Use Insert Time to interpose time into highlighted tracks. 
 
Inserting time adds time at a selected point in your animation, making existing keys slide out of the way. 
Procedures
To insert time into tracks:
- In Dope Sheet mode, highlight one or more item labels in the Controller window to specify tracks for time editing. 
 
- On the Dope Sheet toolbar, click 
 (Insert Time).  
- Drag in the Key window to insert time into the tracks. 
Tip: To insert time globally, highlight the World track, turn on 
Modify Child Keys, then insert time into the World track.