You could run an OVRDBF command from your RPG using QCMDEXC. You’ll need to define your file as user controlled and open the file after you’ve done the OVRDBF.
Or you could create a logical file that is built over all members of the physical file. That way, your RPG program wouldn’t have to know the difference, as the database would present all of the records to the program in the order specified (such as in key order), regardless of the file member in which they’re actually located. If you were to read the records in arrival sequence, I’m not sure whether the sequence would be across all members, or just one member at a time. But you could experiment with that to see…