Hi there,
Good to hear from you again. I see the importance of being able to add custom categories in QuickBooks Self-Employed (QBSE).
At this time, QBSE doesn't offer the ability to add your own categories. For more info on this, I encourage you to reach out to the QuickBooks Self-Employed team using this link: QBSE Support. Let me know if you have other questions. I'll be happy to steer you in the right direction.