How to extend what Registration does?

Maybe I’m a bit late to the party, but we have achieved this by simply extending the existing generated GraphQL input. Here is how our schema.graphql.js looks like:

module.exports = {
  definition: `
        extend input UsersPermissionsRegisterInput {
            fullName: String!
        }
    `,
  type: {},
  resolver: {},
};

Strapi propagates all extended field to User creation, so now we can create users with fullName