Here's how you can add more apps (up to six) to the iPad's dock.
1. Go to your iPad's home screen.
2. Tap and hold any app icon until all the app icons wiggle.
3. Tap and hold the icon you want move to the dock.
4. Drag the icon down and drop it onto the iPad's dock (the other icons will move depending on where you place the icon that you are moving).
5. Push your home button again when you are done to stop all app icons from wiggling.
For more, see the original article at the link below.
How to add more apps to the iPad’s home-screen app tray | Here's the Thing