Please take a look at this note - it doesn't answer your direct question but is a nice read for CP - http://www.arubanetworks.com/wp-content/uploads/aos_guestacccess-appnote.pdf
Now...onto your question, there is no way to edit those embedded pages so your options are
1. Use ClearPass Guest
2. Use a custom html form. This form can use the already embedded page as a template if you so choose.
Use the "view source" option in the browser to see what the default page is doing in the background and then you can upload your edits to the controller as a custom page.
You can install the captive portal page by using the Maintenance function of the WebUI. Log into the WebUI and navigate to Configuration > Management >Captive Portal > Upload Custom Login Pages
This configuration is also found starting on pg 291 in the AOS 6.3 user guide.