You could use the IFrame feature to show your page within a mojoPortal CMS page if you really need to load the full html and CSS.
Alternatively if you just need the content you could either grab the html from inside the body of the html page and paste it into a new instance of our Html feature using the view source/html option of th editor which allows direct input of html. But you should not paste in a head or body or CSS.
Or you could make an html fragment file with just the contents of the body but with the head and body removed (since there can only be one on a page) and then you could include the content on a page using our Html Include feature.
Hope it helps,