Great questions! Currently there is no way to add additional items to a registrant after they have officially registered. You would have to cancel the registrant and re-register them. I can certainly see how it would be useful to add items to existing registrants and I would be happy to pass this feedback to our developers on your behalf.
You are able to change the payment status, though!
1. Click on View/Manage Registrants when on the Event Dashboard.
2. Check the box for the registrant you want to change.
3. Make sure "Payment" is selected in that first dropdown menu above the list of registrants.
4. Select "Paid" from the second dropdown and then hit "Update."