Hello Nargis,
How are you doing,
You question is a bit confusing (as to which order type you are referring) , are you interested in adding extra fields in SAP sales order / purchase order..other orders (service order etc), Then answer will be yes you can change the layout of the order as it was allowed in lower releases.
This part is simple and you can check this link:-
Creating & Adding New Field in sales order
There is another angle to it, your FIori apps for sales order processing which comes with S/4 wont have those extra fields, So you need to get a Fiori developer & a backend ( SAP ABAP & Gateway) developer to enable these fields in your Fiori sales app.
Hope this helps 
& best of luck for you implementation.
Regards
Rishab