I don't understand the question or the answers so I'll try a different tact.
If you are starting from the Order form and you have a new customer who is not in your customer combo and you want to add the new customer and have him added to the combo so you can select him, then, use the not in list event of the combo to open the customer form. when you add the record, set the Response code to record added. I don't have the exact code because I never use this method but you can probably find an example of how the not in list event works.