To import custom fonts to your EPUB books, follow the steps outlined below:

1. Connect your Supernote to a computer via USB. 
2. Create a subfolder named Fonts in the Document folder. 
3. Drag your custom font file on your computer and drop it into this subfolder. 
4. Open an EPUB book on your Supernote.
5. Tap the three-dot More menu (when the toolbar is docked at the top/bottom of the screen). When it's docked at the left/right of the screen, this step will be skipped. 
6. Tap the Display Settings icon . 
7. Tap the circle to the right of the User-defined option.
8. Under User-defined, tap the drop-down menu to select and use your imported custom font. 

Here is a tutorial video for your reference: