You can open a blank EPUB (new book) in the calibre editor (or Sigil) and Import files to add your XHTML files, then you won't have to redo the work because the conversion pipeline won't stamp all over your work.
You can also import a single XHTML file as a new book.
There you go.

Now you can create a book out of "nothing".

(Actually, we just aren't converting it.)