OS/2 is a single user operating system.
When you set USER during the installation, you are setting an environment variable named USER.
How you set up a userid and password depends on which UNIX compatibility software your ssh is based upon.
I use SAMBA Users and Groups to manage my accounts. Another system of users and passwords is accessed by the TCP/IP configuration applet. If you are a Unix user, perhaps the command line apps for user management are on your system. Finally, there is klUsrMgr, which is maintained by bww bitwise works.