Ankur,
I was in an assumption earlier that I create a form and use the form key to somehow render the form in activiti-explorer. If you are trying to do the same, then just forget it. You are thinking in a wrong direction.
You will have to build the UI on your own. When I say UI, the complete UI, right from task inbox, task claim and complete functionalities etc. When you open the task from the inbox, you can get the form variables and read the form key. So at runtime when you read the form key, you can decide on which form to render.
You will get them all if you are using the enterprise version of activiti.