Custom Post Type support is available in the Pro version of PublishPress Series.
With this feature, not only can you use PublishPress Series with WordPress posts, plus you can also create series with pages, or any other WordPress Custom Post Type.
Your series can even have parts across different post types. That means you could have “Part 1” that is a post, “Part 2” that is a page, and “Part 3” that is another post type.
You can enable this feature by going to Settings > Series Options > Pro Features.
Next, go the “Post Types” tab and choose the post types you want to work with: