The users are saved by default under table: jos_users
If you want to use Joomla - you can't store the users in a "separate table" - if you'll do so, Joomla won't "recognize" them as users...
What you can do (if you insist on a separate table) is to create triggers that will copy every change in jos_users to another table.
When you login joomla control-panel (as administrator) one of the options is "User Manager" - that's the place to edit/add/remove users manually (of course you can do it directly in the DB - but I don't see any reason to do it).
If you want to add more options - just add the extra columns to the table!
Hope that helps!