CB Subs Registration Page - Process POST Values

We are using Community Builder with CB Subs on a Joomla 2.5 Environment. We need to pass two values using a POST to the registration page of CB Subs. These two values have to be stored in two CB Subs fields. The values should not show up on the page. Example: we post a Form [login to view URL] We add cb_val1 and cb_val2 to the post request. Values get posted to CB Subs, but are not processed there. We need to display the "normal" CB Subs registration page to the user, but need to store these two values beeing passed in the background with the user record. They are used to identify the device, the user used during registration, so that we can bind / optimize certain services for that device. We need the solution to be delivered asap.