[KevinH]

Sigil does support importing anything that follows the epub spec as indicated by the .epub file extension (ie. filter on the select file dialog). As you noted, simply rename it to .epub and load it.

Is there something in kepub that does not meet the epub spec?

If not, why add a different extension? What purpose does it serve?

I can add it as a recognized input file extension but that extension will be replaced by .epub upon saving in Sigil so what is the point if the file formats are identical.

Would adding .epub but not replacing the .kepub work? Something like book.kepub.epub or book_kepub.epub?

If there are minor differences, then use input and output plugins to handle them. Even if there are no differences, you can use an output plugin for saving it with any extension or extensions you want. That is what input and output pluguns were designed for.