You can actually do this self-serve, without switching to our Upload writing mode.
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.
Update 2021 11 24: Please note that currently, it appears that sample book generation does not work if you are using this process. We are working on a solution.