You seem to be doing everything right from this point. The best way to mix the sound of your X5D and the sound card is through a line mixer. This is hardware, not software. The resulting mix can be recorded by a cassette deck or any other device attached to the mixer.

You could also send the tracks on WinJammer to the synth built into the Ensoniq sound card, but you won't like the sound as much as what comes out of the X5D.