If you have an existing website that has no content management system at all or uses some other content management system that you are not entirely happy with, it can be converted to WordPress. The easiest scenario is the case where there is no content management system whatsoever. The conversion can even be customized in such a way that if you only need to change certain content or certain pages, the conversion can be tailored to give access to only that content which could save in the cost of the conversion. The design and layout of the original site can be duplicated exactly.
In a case where there is already a content management system such as Joomla in place the process is actually a bit more complex. There are various plugins available to import pages, posts and media depending on the system, which may help speed up the content transfer process. But if the design is to be duplicated exactly the wordpress template files should be constructed starting from the generated html. Any existing CSS files can be used as is.
You can also have Wix sites or sites built with other types of sitebuilders converted to WordPress as well. Some of these types of sites call for the creation of a theme starting with a freshly coded layout that duplicates the existing site as closely as possible.