Can we download and add docklets to plank? Is this possible?
If you are talking about adding icons like the Terminal app to Plank, you can either right click the icon when it's open and click "Keep to Dock", or you can find it in the Applications menu and do the same thing.
If you hold ctrl while pressing the secondary click over Plank you can open the Preferences menu, from which you can add some Docklets to Plank.
I don't think there are more docklets available. But if you ask the Plank devs nicely, they might implement more docklets in future versions :)