Without modifying the HC-04 it would be very difficult because you would have to develop a scheme that first synchronizes the two communicating nodes. The reason is that the HC-04 works by emitting a pulse and only then listening for a return, thus it isn't normally listening for arbitrary transmissions.
From the schematic linked below, it seems like it would be possible to tap into the HC-04 "Signal" net and directly detect any received transmission. Then one would have to develop some protocol (OOK, PPM, etc) for message exchange. It's kind of an interesting possibility though the practical value vs RF communications isn't obvious to me. Maybe underwater comms?
