To know more about adduser options visit : Link
Well to change a user from ordinary user to system user use
usermod -s option :
usermod -s /bin/false userName
I would suggest you also to visit : Site
Many times you will have a system where you need a user to exist in the account database (say, /etc/passwd) but don't want to give them shell access to your machine(s). A common solution to this is to set a user's shell to /bin/false. This has the effect of rejecting shell login attempts over ssh, telnet, or other shell-requesting protocols. It may have other side effects too, but those are beyond the scope of this article.
Simply using /bin/false as someone's shell does not keep them from using said account to authenticate over ssh and using non-shell tools such as port forwarding. A default configuration in sshd will often allow tunneling and other non-shell activity.