From SZ version 3.6.2 and above there is a ZD migration feature but as you noted it is for newer versions of ZD code (9.13.3 and above) and creates a new zone with the ZD configuration and migrates the entire ZD's AP's over.
To slowly migrate AP's over time or migrate AP's from older versions of ZD's
Recreate the configuration on the SZ, paying particular attention to any MESH configuration so it matches. Migration of MESH AP's can be complicated. Select the best version of SZ AP code for the Zone.
You can import the basic AP MAC address, Zone (created above), AP name, location and Description using the AP pre-provisioning script on the SZ (under monitor::AP' using "more" drop down list) You can save a sample csv script and fill in the details. Do this before migrating the AP's so they get approved and go into the correct zone automatically. You use this csv to set static IP's on the AP's but having DHCP in the AP management network makes migration much easier.
You could pre-provision all ZD AP's on the SZ but in SZ all AP's included disconnected use licenses. You can get a list of AP's from the ZD Monitor AP using the AP list export button and cut and paste into the SZ AP pre-provisioning sample csv file.
You MUST factory default AP's before they can operate using SZ code
The SZ has a ZD emulation feature that will allow AP's to connect to the SZ as if it is a ZD and get upgraded to SZ code - lwapp2scg. To enable this feature go the the SZ CLI: en; config; lwapp2scg: policy allow-all. Verify by closing config (exit:exit) and run show running-config lwapp2scg
For each AP you wish to move over - access the AP via SSH and: set factory; reboot (note AP's with static IP will need to have dhcp or manually re-configuration); when AP 's back online from AP CLI - set director ip
Instead of SSH to each AP you could use the ZD remote AP cli command from the ZD CLI to run the commands to each AP - en; debug; remote_ap_cli -a "set factory". You need to run this individually for each of the commands above and for each AP. You could create a CLI expect script to automate this for a list of AP MAC addresses.
To migrate MESH AP's you need to start by migrating the furthest MAP in a MESH tree - working into the RAP (Root) AP last. It is essential that the MESH SSID and Passphrase match in the SZ configuration.
It is always recommended to test this on one AP and make sure it is operational before doing any wholesale migration