Dear Namit,
Thank you for your query.
Here is an old description on how to achieve that back in 2009:
http://thebuildingcoder.typepad.com/blog/2009/04/addin-keyboard-shortcut.html
Things have changed since then, though.
The Revit 2011 API introduced keyboard shortcut support for API buttons:
http://thebuildingcoder.typepad.com/blog/2013/02/whats-new-in-the-revit-2011-api.html
Keyboard shortcut support for API buttons
API buttons found on the Ribbon can be assigned a keyboard shortcut. Buttons created by applications registered using manifest files now use an id based on their application id and button name to provide a unique identifier for the button. The keyboard shortcut will be maintained even if the order of registration of API applications changes.
This functionality was overhauled in the Revit 2013 API:
http://thebuildingcoder.typepad.com/blog/2013/03/whats-new-in-the-revit-2013-api.html
Support for Keyboard Shortcuts and Quick Access Toolbar
API commands may now be assigned keyboard shortcuts, and those assignments will be preserved even if add-ins are added, removed or changed in subsequent sessions.
API commands may also be moved to the Quick Access Toolbar, and that assignment will also be preserved even if add-ins are added, removed or changed in subsequent sessions.
I hope this helps.
Please take a look at that and let us know how you can make use of this functionality.
Thank you!
Best regards,
Jeremy