You should just be able to copy and paste your css and js into a Resource > Stylesheet or a js Code > Script Library. Then you can just use those resources as you normally would in xpages. Typically, when I have done that, I have renamed the resource. So, for instance if I was using a page to store my css called style.css, I just named my new xpages stylesheet xspStyle.css. That way you don't touch the legacy stuff while you are redeveloping to xpages.
Also, as far as the html goes, you can pretty much copy and paste your existing html between the <xp view> tags in your xpage and it will be fine. You'll probably then need to go in an tweak it to get it to work/display correctly, but it beats starting all over from scratch. If you are using <Computed Value> in your html, that will all need to be removed and redone. By and large, the more strictly you adhered to html conventions the easier it will go for you.