Not sure if i understood you correctly, if you were asking how to assign Rsh to a userID, here's how it can be done:
Well, you can use smitty user, select the option to show/change user characteristics, you can see the Initial Program -> you can point it to /usr/bin/Rsh instead of the default ksh.
(Be sure to check if /usr/bin/Rsh exists in your system)
You may also want to do the direct way by just modifying the /etc/passwd file, the last entry in the line delimited by ':' to the path /usr/bin/Rsh.
Below shows an example for restricting userA to the designated ftp directory in /etc/passwd file:
Hope this helps.