At the moment if you go into the Amend a Sales Order screen if you edit a product line you are not able to select the Suggest Cross Sell products. This means that if you have started the Sales Order as a Rapid Entry order and then convert to a Full Order entry this functionality is not available to you.
The cross sell option is available to new lines within amend order, but not existing lines. Is the requirement to add for existing lines on an order?