Hi Harish,
Try following settings in your Header material master.
Set procurement type = X
Special Proc key = 20
Now if your demand is greater then your opening period defined in Schedule margin key, system will generate Pur req within opening Period and Planned order outside the opening period.
Components assigned under Header materials will behave according to the material master settings and planned with MRP run if "Also plan unchanged components" check box is selected during MRP run.
Regards
Nitin