MAAS does not currently support custom Ubuntu images. See Creating a custom Ubuntu image for the reasons why as well as a suggested method to create a custom image. Unfortunately the kernel is part of the reason we don’t support custom Ubuntu images.
The supported method to perform Ubuntu deployments with a static set of packages is to mirror official images, Ubuntu archives, and configure MAAS to use your mirror. Once configured MAAS will reconfigure the image to use your mirror in the ephemeral environment and during deployment. You can then control when the mirror is updated to ensure the package set is static.