Can I upload my own manuscript as a book of a series?

Yes, from version 1.9.7. When you import a manuscript, the "Series (optional)" field lets you make it the next book of an existing series or start a new series with it. You can also open a series page and click "Import a manuscript as Book N". Right after the import, BookyAI prepares the book for the volumes that follow: it writes a continuity summary for every chapter (the "story so far") and, for fiction, maps the characters into the series codex. You can watch the progress in the book's workspace; the Series card shows when the preparation is done. The next AI-written volume then continues your book rather than only knowing its chapter titles.