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.
Can I add Self Assessment to my QBO essentials package?
Yes I am referring to adding QBSE to my current subscription of QBOnline Essentials, surprised to learn it’s not a plug-in. If I purchase a separate subscription of QBSE in addition to my current QBO subscription can I import/batch transfer the relevant data from QBO to QBSE to auto populate my tax date for returns etc. or do I have to type in the relevant data from QBO to QBSE manually?