Hey, Prasen,
Yes, you can re-open an existing add-in into Add-In Builder, but it's a little hidden. If you do File > Open and navigate to the .jmpaddin file and select, then look at the Open button at the bottom of the dialog - it is a "split" button with an arrow that brings up a menu. On that menu is an option to re-open the add-in in Add-In Builder.
This assumes you are running on Windows. On the Mac, I believe there is a check box you can check to re-open in Add-In Builder.
HTH,
Eric