With MIGX 3.0.0-alpha5 on MODX 3, you can’t use the package manager or class files created in MODX 3 (schema version “3.0”).
It should be possible with MIGX 3.0.0-beta1, but at the moment you have to load the package from here:
MIGX/migx-3.0.0-beta1.transport.zip at master · Bruno17/MIGX (github.com)
Another possibility is to create the class files for the custom table in MODX 2 (with schema version “1.1”) and then move them to a MODX 3 installation.