Hi there, duncan-robertson,
Thanks for posting your question in the Intuit Community! I'd be happy to answer your post.
If you’re referring to adding QuickBooks Self-Employed (QBSE) subscription into your QuickBooks Online (QBO) account, it’s currently not an option since these products are two different versions of QuickBooks.
You can check out QuickBooks products comparison for more information: https://quickbooks.intuit.com/uk/pricing/.
However, if you mean something else, please let me know. I'm always here to answer it for you.