You can either do an OVRPRTF before you call the program which opens the printer file (you can even do that from within the program itself by calling QCMDEXC, provided the printer file is user-controlled and it's closed when you do the override).
Or, if these are specific printer files, and you are 100% sure that everything that prints from that printer file goes to a specific printer only, you can CHGPRTF and chage the outq for that printer file.
"When once you have tasted flight, you will forever walk the Earth with your eyes turned skyward, for here you have been, and there you will always long to return."
--Leonardo da Vinci