The very first Performance in Montage is an example of this very thing - although it uses acoustic piano and electric piano. Start there.

"CFX + FM EP"

CFX = Yamaha's top of the line premium acoustic piano
FM EP = Yamaha's best selling DX7 style Electric piano.

When you first call it up, you hear just acoustic piano, you must move the Super Knob to "morph" from acoustic to electric piano. So can the Montage do this? Yes I think do 🙂

You can plug a Yamaha FC7 pedal into the Foot Controller 2 jack on the back panel - the Factory Performances are programmed so that this automatically controls the Super Knob.
Heel down = Acoustic Piano
Toe down = Electric Piano

There are many examples of this type of transition between sounds.
We have constructed a series of articles here on the YamahaSynth website that will help you get into programming the Super Knob to transition between sounds.

In Depth Montage Programming Series

Thank you, Bad Mister.

If I want to add Strings pressing Volume pedal, keeping Piano, is the same way to do it? Not transition between sounds, I have piano sound, i want to keep it while a I press mi Volume pedal to add Strings (But Strings should has Volume 0 by default)

If this question is alaready answered in you response I will check you information. 🙂

Thanks for your help!

Best regards,
Rubén

Ah, I understand. Please see this short video by Hans-Peter

https://youtu.be/bUyKeTQ8LPM

Basically, he starts with a piano
Adds strings to Part 2
Sets string volume = 0
Assigns the Super Knob to control Part 2's Assignable Knob 1 (Part 2 Assign 1)
Edits Part 2's Mod/Control > Control Assign so Assign 1 is set to PART VOLUME
Sets the CURVE = Standard
POLARITY = Uni
DEPTH = +32

Super Knob now controls just String Volume, Piano remains.
FC7 controls Super Knob movement.

Yes, Bad Mister, Thats it!!

Thanks for your help, you have been a great support,

Best regards,
Rubén