This is not possible. I don't know what NAV version you use, but you can update Sales Order when you want it. If I understood your scenario, you created Sales Order from Sales Quote. I will give you example.
I created new Sales Quote with two lines; first line is 100pcs of item A and second line is 200pcs of item B. When you run command 'Make Order', you will get new Sales Order.
After that, you can change any line in order or delete/add new line. If you have some shipped or invoiced quantity, you cannot modify this line.
If you have line with no shipped or invoiced quantity, you can modify this line. Probably, you have order in Released status. If you have this situation, you need to Reopen this oreder and you can add/modify/delete lines.
This is standard NAV procedure. If you have some other problem, please give us scenario with more details.