You can use Session Variables in your application however this is not recommended, as they will create State in your application. This will cause problems in the case that you plan to scale your application to more than one partition as Apprenda does not handle sessions across machines. By default, Apprenda does provide server affinity, but this is not guaranteed. You can use our Distributed Cache Layer to store information across all servers.
With regards to modifying the User Tables, the best way to approach this scenario would be to extend the OnBoarding screen so that it takes you to a secondary application developed by you, in which additional information can be obtained and stored.