Localhost Regisered users

Right now I’m working on a site offline. I’d like to make some customizations to the registration process(if possible), but I’ve run into my first issue. I can’t seem to complete the process as a guest registering offline.

Is there any way for me to create a registered user that can login manually?

Also is it alright to add additional table fields to the database that would take more information from the registration form?

Thanks
Chris