Hi Hitesh,
Did you checked the Offset field available next to requirement date through which you can manipulate the date populated due to scheduling configuration based on the order start and end date.
Say if you enter 10 Days in the offset field,requirement date say 06.02.2014 will get changed to 16.02.2014.
Making requirement date field editable is not possible through customizing and may be possible through user exit(once we did for currency field in operation details) but it is not advisable to change the STD SAP behaviour.
Thanks
Girish
