You can issue an ovrdbf command in a CL prior to calling the Rpg program.
Assuming RPG IV in a reasonably recent release:
F someFile IF E K Disk infds( someFileDS )
F extfile( MYFILE )
F extmbr( MYMBR )
D MYFILE c const( ‘MYFILE’ )
D MYMBR c const( ‘MYMBR3’ )
The EXTFILE() and EXTMBR() keywords can refer to literals or variables. If you use a variable, you can close the file, change the variable value and open the file against a different member.