If you can find out about the PPM voltages on your radio, find one of the "bigger" makes that is compatible, you can buy a DSM module for that make (like the Orange ones) and make a connector to connect it via your trainer port. If you do not have a trainer port then just open up the radio and make a connector to the Vcc, GND and PPM connectors of the internal module (remembering to add a 2P2D switch to the internal module to deactivate it when using the external one) and connecting that to the external module.
This is all really only a good idea if you're a bit of a tinkerer, because if not, your cheapest option might be to get the Orange 6ch Tx.