'Rendering' MIDI means using a software synth.
Since you are using a Hardware synth, you would need to record your external hardware synth to additional tracks as Audio so they can be Rendered that way.
So yes, Realband would be your better option.

In BiaB use Save As to give the file a new name (and thus protect the original).
Freeze all the tracks and Save again.
Open the resulting BiaB file in Realband, let it go through it's generating process for a minute.
Play it, and if acceptable, Save As a SEQ file in Realband.

Then Record your hardware synth to new Audio Tracks in RB.
Save again.
Once this is done, you can then Render the song to a stereo wave file ..

The other option is to record the BiaB song in realtime to a different Software, but this can get complicated, depending on the system configuration and available options.

Maybe others have better ideas, but that's how I would proceed.

Make your sound your own!
.. I do not work here, but the benefits are still awesome