While we do want to be able to include the option to import user fonts, this is a complicated issue and it might be a while before the feature is added.
Incorporating this feature is complex due to the legal requirements associated with font licenses. Each font license has specific conditions that must be followed before it can be embedded into a file because this process effectively involves sharing the font file along with the book, which may violate certain font licensing agreements. Each font licensing agreement may be slightly different, and have different requirements for this embedding process.
We want to ensure that both our authors and the creators of fonts are fully protected before we allow for importing fonts. While we aim to include the option to import user fonts in the future, it may take some time to address these complexities.
In the meantime, best workaround for user-licensed fonts is to create your chapter titles in e.g. Canva, BookBrush, or Adobe Photoshop, save these as images and then add them to your chapters as custom header images.