Hi Arif,
it seems surprising that the customer wants to assign all of the tasks to the same assignee and group, can you give more detail about what they are trying to achieve?
If the WF tasks belong to a change category workflow, you can create a 'behavior' for the status Pending for each task in the category, that will set the WF task assignee and group from the CHG assignee and group when the task goes Pending. You will need to create a macro to assign the values, but the macro only needs two lines of code.
Regards,
james