If I read you right it might be doable. You will need a 5 pin MIDI to an USB converter:

https://www.amazon.com/TraderPlus-Converter-Synthesizer-Interface-Controller/dp/B079SWVRXT/ref=sr_1_1_sspa?ie=UTF8&qid=1536881558&sr=8-1-spons&keywords=midi+usb+adaptor&psc=1

They cost $11 USB. Plug that into your computer and select BiaB to export MIDI to it. I do this only in my DAW so I have no idea how to do this in BiaB.

Now if you only want to hear your hard synth then you can run your outputs to your amp and speakers. But if you want to record that synth then you will need a way to get the audio into the computer. If your synth is mono then you are all set. Just run the synth audio output into your into your mic jack on your audio interface, this assumes that your mic input will also take a 1/4" plug.. But if the synth is stereo and you want to keep it stereo then you will need an audio device with two line inputs. If you also want to record your vocals and guitar you will need an audio interface with enough inputs to accommodate everything.

Or you can do it in a DAW. Record your MIDI, then on another track record your guitar, and on another track record your vocal. Again if your synth is mono you may be all set to go now.

Personally I would do this in a DAW; remember RealBand is a DAW and it is capable of doing this. I have very little knowledge of RealBand as I use another DAW so someone else will have to help you with this.

Good luck.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware