Conversion plugins without input plugin

I am planning on creating a calibre plugin for my epub to kepub conversion tool kepubify. I would like to be able to skip the input plugin stage for epub inputs, so I can keep the fast conversion speeds (kepubify is about 12-30 times faster than the kepub output plugin). Basically, the plugin would run a standalone executable on the epub file, which results in a kepub file.

How should I go about creating this plugin? What type (conversion or just a toolbar button) should I use? And if it is a conversion plugin, how would I skip the input plugin stage for epub files.

I am new to calibre plugin development, but I already know python quite well.

Thanks in advance for the help.