@spearsn EDIT: Can't be done if the vendor was retail channel. If it was an institutional vendor, and they are currently capable of enrolling devices, they should be able to. I have had Macs added to ABM over a year after purchase (once the vendor figures out how to add previous purchases it's a much easier process to get them to do it again).

The reason you had to release your leased Macs from ASM when they were returned was so whoever they will be sold to (Apple doesn't lease used Macs AFAIK) could re-image them without being automatically enrolled with your Jamf Pro instance. Any org that uses ASM, or the commercial equivalent Apple Business Manager, should have a disposal process step to release Macs for the same reason.

@spearsn I would suggest going back to your vendor again. Apple allows vendors to retroactively add macs to a customer's ASM/ABM account that they had previously sold. As long as there is a valid chain of custody the can add it. Now, it is up to each vendor whether they are willing to retroactively add it or not. But, i would push back and question why they won't do it.

Thanks for the responses guys, very helpful. I've tried a few times and my emails just go unanswered. They are all chatty with me when I need to purchase from them, and then get ghosted when I bring this up.
I was watching some JNUC presentations and saw some chat about using Apple Config. 2 to add devices so I thought about looking into this again.

Vendors can do it, yes.

One of the things though, the vendor has to have specific (purchase related) records to be able to do it. Our primary vendor does not "store" the information/records in accessible data space going back far enough for us to add some of our older devices unfortunately.

So that said, it's not simply simple, i.e. it's not trivial, but if they do keep the required information in an accessible space, it shouldn't be too difficult for them, once they've done it, as others have mentioned.

Vendors can add them, they need the Apple Purchase order number for the device. Then they can add them in. Reach out to your Apple Rep and get them to find the number for you, and give that to the Vendor. We have done it this way before. Now all of our Vendors are under instruction to add everything they sell us, before it gets to us.

You can use Apple Configurator to retroactively enroll any iOS device (though with a limitation : it installs a profile that can be removed for 30 days, and only after this 30-days delay is over, the device is permanently enrolled in Apple Business Manager).

However, you can't use Apple Configurator for macOS devices. It's iOS-devices only. Other answers are right: only through a select ABM-capable vendor you can retroactively add you macOS devices in ABM, if these were bought through the proper channel.

Speaking entirely theoretically, without any hard evidence to support this hypothesis whatsoever... there's a chance that potentially Configurator may at some point gain the ability to enroll Apple Silicon based Macs, much as it does with Apple Silicon based mobile devices. That's about as far as I think Configurator will go; I strongly doubt it will ever gain the ability to enroll Intel-based Macs.

iOS, iPadOS (just to specify), and tvOS can be added via Apple Configurator. It's only macOS that cannot (nor watchOS...which isn't "manageable"...yet(?)...)

I can see what georgecm12 is saying becoming a reality some day, following the same restrictions are currently present on other devices mentioned by ggetenj (30 day user removal window as well the device must be wiped to initiate the process).