The only way you can do this is to incorporate a form within the product (other than having them email or call of course).
Keep in mind that the form does "not" integrate with the shop so it is a totally separate entity. If someone were to fill out the form, the person on the receiving end that takes the orders and the information for customization would need to match up the customization form to the order to be sure that:
1. There is an order at all.
2. That it goes with the right order.
The other thing about it not being integrated is that the customers should be aware that it's sending a separate email that has nothing to do with their purchase so they don't wonder what the deal is.
The form can be added to the page using the shop's HTML tool. I would look around in the articles here on the forums for exactly how to implement it as I can't remember now, it's been quite a while since I've used the program. You should be able to go to your Help area in your account here and on the right hand side there should be links to various locations to find articles on SCCP and you can search them as well.