would be easy with a wp plugin. in the post text, just put something like AFFID where you want the affiliate id. then with the plugin just do a str_replace("AFFID", $_GET['affid']);. (of course, sanatize the $_GET['affid'] to numbers only (assuming ccbiill).
There are probably text replacement wp plugins out there that will do this.
(and you can just do <a href="site.com/affcode=<?=$_GET['affid'];?> in your template for banners etc. (again, make sure you strip anything but letters/numbers from $_GET['affid'];)