Create your slideshowm in keynote.
Export it as HTML.
Put the exported folder on the server.
Display the slideshow in a browser.
When it works as intended, go to iWeb.
Add a page for your slideshow to display.
Add a HTML Snippet.
Use the URL of your slideshow as src="" or as location.replace()
For keynote questions, consult the keynote forum.
Or you can export it as a QT movie and add the movie to an iWeb page like described here: HTML5 Video tag to play movies on your website. You might be able to retain any special effects, transitions, etc. if you conver to a movie. You'l have to try both methods and see.
Here's an example of a Keynote exported as a movie and displayed using HTML5...
This preserves the 3D transitions.
Leynote exported as HTML...