I'm Alex, with the Shopify team!
Thanks for reaching out! There are some limits to the extent to which you can customize your checkout, so an entirely new field can't be added to collect that information from your customers.
If ID is something that you'll require from your customers when they purchase from you, what I'd suggest is making customer accounts mandatory when they purchase from you, and including a custom field in their registration form. We've got a great guide on doing so right over here - this would save your customer's ID as a note in their profile within your admin!
Hope that's helped! Please feel free to let me know, and you can always give us a call or start a live chat at any time. We're open 24/7 for your convenience and always happy to assist!
Alex | Shopify
|30 seconds ago|