Insert Sales Order Item into existing Delivery Order

You may go to tcode VL02N and go to->Outbound Delivery->Deliver Sales Order to include your item again in the DO.

Insert your sales order number and item no.

Disallow to insert item manually into Delivery Order without Sales Order

Go to tcode OVLK and do the below setting as higlighted.

Change Posting Date for Delivery Order

If you want to change the posting date for delivery order, please change it using tcode VL02N and follow the below steps.