You can find RF Modulators at some places. They connect to the ant. in port and override a signal on a certain station when it's in use. Old CD Changers used to connect to stereo's that way. You can also buy the FM Modulators that do basically the same thing, but, I bought one and it gets overpowered by the over-the-air waves. The RF Modulator blocks out those signals while it's operating.
OTOH, it may be cheaper/easier to just buy a new stereo w/ aux in, as it's sometimes hard to find RF Mod's that accept RCA in.