I think you should reconsider your plans.
There really isn't much benefit to using OpenACS without the users login and security system. By writing your own you can also introduce security issues that have been discovered and resolved over 10 years of use.
If you want to extend the information you collect on registration this should not be too hard. The first question is what version of OpenACS are you using. From there I can recommend the best solution.
If you want to understand how sessions work you should review under openacs-4/packages/acs-tcl/tcl/security-procs.tcl.