Hi neil39,
It's great to see you reaching out to Community! I can understand the benefit of having custom categories applied to your expenses for tracking, recording and reconciling purposes. However, QuickBooks Self Employed currently doesn't have the option to create custom categories. Please feel free to submit any suggestions you may have to our Development Team. You can do this by going to the Assistant menu and enter Feedback and continue to follow the prompts to share your ideas.
If you have any other questions, please reach back out. We're here for you!