Hi Equipe,
Its not possible to enable these fields(fields not availble in field status settings for operations) as editable since these dates are derived based on the basic start date and basic finish date at the order level(in the main screen of the order) and system propose these dates as per scheduling parameters customized per order type.
You can change at order level which will adjust the operation dates but this will be applicable for all operations in the order.
As Patric mentioned already the only way is to use the restiction feature which SAP has given to manipualte the operations dates which will allow you to do operationwise.
OR if it is critically required ,you can open up these fields using userexit with the help of ABAP person but it is not advisable to open these standard SAP fields.
Thanks
Girish