I am not 100% positive, but I believe if you do a firmare update on your DX8 it may be DSXM capable. It's been a long while since I've messed with any of that.
I remember reading that the early radios were DSXM capable just not labled as such, but needed a software upgrade to enable it. If you check the Spektrum website they should have firmware updates available. It's worth a shot. (Especially since it won't cost you anything) If not you will need to buy a newer transmitter.