You can actually do this self-serve, without switching to Bring Your Own Book.
The steps are:
- Make a folder called "custom" in the root of the Dropbox folder for your book (it should be a sibling to your manuscript folder
- Put a file in there called "SLUG.pdf", where SLUG is the name of your book in its URL. If you have a sample as well, you can put a file called "SLUG-sample.pdf" in there too.
- Re-publish your book.
We will then generate the EPUB and MOBI files for your book, and use the PDF files you have provided.