Hi Joy,
I hope when you meant activiti, you mean to say activiti-explorer. You cannot link a jsp to a form in activiti-explorer. The way you have to go about this is by implementing your own inbox in your application and your own jsp form to load when that task is opened.
You can use the below API call to get the list of tasks assigned to a user
processEngineInstance.getTaskService().createTaskQuery().taskAssignee(userName).list()
When you have the TaskID of the task which was clicked, you can easily get to know its form properties where in you can have your form name defined there. So based on the form name you will have to load your JSP.