adding a custom field in the checkout workflow?

Hallo,
I managed te make a custom field for selecting a deliverydate for a produkt.
But when a customer buys two produkts he or she can select two different deliverydates.
That not so handy i guess. Is it possible to place this field ONCE for each order.... in the checkout workflow?
I tried to make custom fields in different types put it does not show up where i want.
So, is this possible and than...how?

Thanks!

I found out how it works! Just made a custom field with table: order. After that i had to place the field in the checkout workflow and it works great. I'm lovin it.