Strive does not allow additional statuses to be added by design.
If you need another status, it is most likely a task and should be added as a post checklist item instead.
For example, you might want a status called “Unassigned” for a post that hasn’t been assigned to an author yet. However, posts in this status would change statuses immediately upon completing a single action: assigning an author.
For this reason, it would make more sense to add a task called “Assign an author” to the Not Started section of your checklist.
Using the post checklists feature, you can organize every step of your content creation process by status.