Store custom signup fields

I’ve been researching this for two days and now I give up, please tell me how to achieve the following:

I have custom user fields added to wp-signup.php through signup_extra_fields. Among those is a file field for user photo upload but I’ve fixed the multipart/data problem via JS.

The user fills out all the fields and hits “Register!”, after this I want to do the following:

1) Extract my custom fields data from the $_POST and store in database
2) Link the new user’s id to the new post in my database table

I know how to write the code to do 1) and 2) but I don’t know what WP action to use to be able to access the custom data before that stuff magically disappears from the $_POST (by the time I reach signup stage 2 it has disappeared).
Which add_action(‘hook_name’,’my_intercepting_function’) should I use?
Or do I need to use a filter and if so, does that need to be installed with the plugin or can I add it in the plugin PHP-file later?