Free of goods

Dear sap sd experts

I would like to know one thing

Generally in free of goods Free of goods will add automatically to main item

if we maintained like this : Main item A  --- And free of goods B

Now if we create Directly B item in the sales order( with out main item) system will price For B item ( it is normal Business process)

But my requirement is For some sales documents if i create B Material price wont come it will be free of cost

I know we can change Item category From TAN TO TANN But i want directly