All it takes is basic CSS. You can create a blank file, move it to where you need it, and just fill stuff in in the body tag.
You can also liberally copy-paste from the other files.
Most likely all you will be doing is inserting some
<p>paragraph text</p>
sections, maybe with a style you liked from elsewhere in the book.
Alternatively, if your master copy was a DOCX, you can update that and regenerate the EPUB.