Virus TI Multi mode: layering Virus patches AND external MIDI signals

[Alakhai]

Hello everyone
I'm setting my Virus TI keyboard for an EBM live-band
I'd really like to use it as the main synthesizer, but having two other assets (a MicroKorg XL and a Novation XS rack) I'd really like to use it as controller for the other two.
Specifically: for a single song, I have a single Multi patch, using 3 parts for 3 different sounds on 3 octaves (there's no need for extention, I have to play robotic melodies on and on ) so I still have 2 "free" octaves. is it possible to use some external MIDI device (with standard MIDI cables) ONLY on the remaining two octaves? I mean: a split like this:
oct 1: part 1 - Virus program 1
oct 2: part 2 - Virus program 2
oct 3: part 3 - Virus Program 3
oct 4-5: part 4 - external synth - program 1

I already tried to disable "keyboard to MIDI" on part 1-2-3 and enabling only part 4, but no use - the external synth kepps on playing from octave 1 to 5!

I'd like to use MIDI channel 1 with all the parts, to avoid the need to "switch" channel while playing live. is there a way to do it?

I hope you'll understand my question, sorry for my bad english XD

Cheers and beers
Alakhai