Storing data for users

I figured out a better way to do this.
In user.register.post.post_process:

var ssn = event.request.payload.ssn;
var user_id = platform.session.user.id;

platform.api.patch("system/user?related%3Duser_lookup_by_user_id", {
    "resource": [
        {
            "id": user_id,
            "user_lookup_by_user_id": [
                {
                    "name": "ssn",
                    "value": ssn,
                    "private": false,
                    "description": "The SSN of the user."
                }
            ]
        }
    ]
});

The above code sets a lookup key for the newly registered user.