HubSpot's default lifecycle stages cannot be customized. These stages are used in analytics within HubSpot to give you a clear picture of your contacts database. There are also properties (e.g. Became a Customer Date) that are automatically created and updated based on the default stages. If needed, you can create a custom contact property to store customized values. Learn how here.
Professional and Enterprise Marketing customers can also use workflows to automatically update the HubSpot's Lifecycle Stage property when the custom property is updated.