Add apple devices to MDM portal

Just to clarify, you appear to be talking about two different services but interchanging the terminology. The MDM manages the devices. ABM or ASM (I will assume you are a business so I will use ABM from here on) is about establishing chain of custody and starts at purchase by using a DEP capable reseller. ABM is where you exchange your reseller and customer IDs. But this will be for new purchases. Previous purchases are not eligible for automatic ABM assignment likely due to the incorrect purchasing procedure - you purchased via retail and without exchanging reseller/customer ID.

If you have existing devices, you have three choices with most MDMs. You have not provided the MDM you are using, so I cannot provide exact guidance. However, most MDMs have the ability to perform user-initiated enrollment. This is done by going to a web URL and following the MDM vendor's procedure for manual device enrollment. For example, this is Jamf's documentation on User-Initiated enrollment.

A second option is to use Account-driven user enrollment. Again, here is Jamf's documentation on this method. This process requires the creation of Managed Apple IDs. But, MAIDs open up a can of worms for most organizations. The best way to implement is via Federation and Sync. But, that can be tricky as Apple IDs using your company's domain will need to be migrated. This is the rarest method we see.

The final option is to use Apple Configurator to side-load the existing devices into ABM. Note, this is the most disruptive as the devices need to be erased in order to inject into ABM. All data will be lost on the devices. However, this method can convert compatible, pre-existing devices, into ABM assets and thus eligible for automated enrollment (full supervision). Note, Macs must have a T2 controller chip or better to be eligible for this action. If you have Intel models without T2 chips, you likely should be replacing them anyway. If that is the case, make sure you are buying the replacements via the proper procedure to ensure automatic assignment of the new hardware.

With the Apple Configurator side-load, you will be able convert existing devices into chain of custody linked assets. They will now benefit from automated enrollment, supervision, and remain in chain of custody until released by your ABM admin.

Bottom line, if you want the fastest way to managing existing devices, use the user-initiated enrollment procedure available from your MDM. You will not get full supervision, but you will at least be able to start central management. If you have the luxury of wiping the devices, use Configurator to capture the assets and then redeploy them.

Hope this is helpful.