Since this viewer uses MuPDF as the document engine, it should already support EPUB, MOBI, FB2, and XPS document formats.
If it doesn't, it should be a relatively simple task to upgrade the mupdf library to the latest version and recompile Sioyek to get all the file formats supported by mupdf.
If it doesn't, it should be a relatively simple task to upgrade the mupdf library to the latest version and recompile Sioyek to get all the file formats supported by mupdf.