Syed, Yes - this is achievable by leveraging the Aruba ClearPass Guest registration portal but I do not see a means to do this based upon the InternalDB configuration of either the controller or IAP based internal CP capabilities.
From the ClearPass Guest Deployment Guide found here:
http://support.arubanetworks.com/DOCUMENTATION/tabid/77/DMXModule/512/Command/Core_Download/Default.aspx?EntryId=10987
<snip>
The Aruba ClearPass Guest self-service portal is accessed through a separate link that must be published to guests. The page name for the portal is derived from the registration page name by appending “_portal”.
When the self-service portal is enabled, a Go To Portal link is displayed on the list of guest self-registration pages, and may be used to determine the URL that guests should use to access the portal. The portal offers guests the ability to log in with their account details, view their account details, or change their password. Additionally, the Reset Password link provides a method allowing guests to recover a forgotten account password.
</snip>

If you are not familiar with our ClearPass Guest product, you can request an eval thru your friendly neighborhood Aruba SE :)
While I'm sure you could write an interface to connect to a home-grown captive portal with a AAA backend including the means to change the password of a user - why re-invent the wheel? ClearPass is core the the Aruba Product portfolio and the feature functionality exists today.
Cheers, Adam