It is possible for Apple to do it for you. They have an internal order number system, and they need to find those numbers. If you give the serial numbers for the Macs to your Apple Rep, you should be able to get them to find the Initial Apple Purchase order number. Then you should be able to press them to add them to your ASM. They really do not like doing it. The Order number thing is the key, get that for each and then move on to get them added. In theory any, really nice kind helpful, reseller can add them with the order number details.
If you look in your ASM and existing assignments you should see the Apple order numbers for their entry.