You can create custom fields on existing objects, but you can't create new types of object inside Netbox.
You also can't extend the Netbox DB schema. If you do, then you've forked the project and you're no longer using Netbox - you are the developer on your own, with nothing but the Django documentation to fall back on.
Better to create a separate application and then integrate it with Netbox - e.g. using the Netbox API and webhooks.