AS/400 Tips 18

POP UP COMMNAD LINES
It's pretty common to be several programs deep in an application when you suddenly have the need for a command line. There's any easy way to allow this without having to back out or switch to a different session.
Program QUSCMDLN is the IBM supplied API to call a command line. No parameters are required. You may want to add a function key to your application programs (perhaps F21) to call this program.
If you don't want to go through this much work, simply change your user profile (CHGUSRPRF) and set QUSCMDLN as your attention key handling program.

Source: http://midwareservices.com/tipstricks/command_lines.htm