What does it matter just getting users to type `su - username` in the shell, if they must type their username and password in any way?  Only 5 extra characters typing :).

If it's not acceptable by users and you are so lazy as not to want to write single script anymore, just install `telnet` and `telnetd` on the server and put the following alias in `/etc/bash.bashrc`.

    alias x='telnet localhost'

**Updated:**  Don't forget to restrict remote accesses to telnetd port (tcp/23).  You can set up firewall (`iptables`) or traditional TCP wrapper (`/etc/hosts.allow`, `/etc/hosts.deny` - consult hosts_access(5) manual page).