The easiest way is to add a workstation entry to your other subsystem and specify workstation names for the users that you want to run in that subsystem. For example you want all your accounting users to run in subsystem ACCUSERS.
Make sure that all your accounts users have workstations that begin with ACC (ACCDSP01, ACCDSP02, etc).
Add a workstation entry to subsytem ACCUSERS :-
ADDWSE SBSD(ACCUSERS) WRKSTN(ACC*)
Now all the workstations that start with ACC* should be allocated to the ACCUSERS subsystem.
Just to be clear – you cannot move them during their interactive job.
Gilbert’s method would take effect the next time they connected.