How to convert HTML pages into wiki pages?

XWiki supports HTML as content of pages. See Page Editing.

Once you have the content of a page saved as HTML, you can then edit the page again and change the page syntax by choosing XWiki Syntax 2.0+ and the content will get transformed into xwiki syntax.

Note that hyperlinks will not get transformed into relative links, they'll stay absolute. You'll need to manually edit them to make them local links.

An alternative to all this is to create a new page, edit it in the WYSIWYG editor and use copy paste the HTML in the content, and then save.

If you need to script conversion from HTML to XWiki Syntax (for example you need to convert a batch of content from HTML to wiki pages) then you should use the Rendering Script Service. To understand the Rendering architecture, check the Rendering wiki (here's an example for converting content from one syntax to another).