Until there exist a program that can directly convert CRYPT6 to HTML you have to use a workaround to achieve that. In order to convert the encrypted chats from CRYPT6 archives to HTML, you first need to convert crypt6 to crypt and then use one of the available crypt to html conversion tools to encrypt and extract the .crypt archive and save it as .html file.
The latest version is CRYPT12 so this conversion is a little outdated.