Dimple

You can; it's called Dimple. About the size of a small Band-Aid, this deceptively simple sticker adds four programmable buttons to Android phones and tablets. Just apply it to the back of your device or case (as long as it's non-metal), then use the Dimple app to set up the buttons.
There's only one catch, at least on paper: Dimple requires near-field communication or NFC, a feature not every phone or tablet has. (NFC World has a fairly comprehensive list of those that do.)
On the flipside, if your device does have it, you're golden. Dimple requires no battery of its own, instead relying on the magic power that emanates from NFC. (Who knew, am I right?) Similarly, it utilizes NFC for its very functionality, not Bluetooth or Wi-Fi.


http://www.cnet.com/news/dimple-nfc-sticker-adds-four-physical-buttons-to-your-android-device/