Can I use external midi Knobs for FM-X parameters?

Yes, Here’s how: The MONTAGE allows you to assign certain FM-X parameters (29) within its massive Control Assign matrix. (See Data List booklet 182-184) for a full listing of 251 potential Destination parameters)
By assigning any of the 29 assignable FM-X parameters to a physical controller you can then manipulate that parameter by using the associated CC number to control the parameter.

If I edit part 1, Op1, highlight level, attack, press Control Assign and then send a CC message in on Ch1, will that work?

No. That is not how you make the assignment.

If you want to assign the Op1 Attack parameter as a controllable parameter you must assign it to a MONTAGE controller, (highlight the parameter you wish to assign, press [CONTROL ASSIGN], move the MONTAGE controlller), then assign your external device to control the MONTAGE controller by assigning it to the same CC that the MONTAGE is assigned to transmit from that controller).

For example, if you set OP1 AEG ATTACK to be controlled by Assign Knob1, then sending CC17 In via MIDI will control whatever Part Assign Knob 1 is set to control... “OP1 AEG ATTACK”

Thank you. :-)
(I shall see if this is on ideascape and if not, post the suggestion there.)

You realize that what I told you works?? Right? but you must move the physical control (the MONTAGE controller) to assign it to a MONTAGE parameter, then you just assign your external controller to the same number. Again: the MONTAGE does not address itself with MIDI... it addresses itself directly. Rather MIDI is generated by the selected physical MONTAGE controller. By assigning the PHYSICAL controller you determine which cc can be used.

Moving an external controller will not register to make the assignment.

1) assign a MONTAGE controller
2) then assign you MIDI device to that control number.

Many (many) users mistake a MIDI event coming in from an external device and a direct event generated by the physical hardware. Although they can accomplish the same thing — they are NOT the same thing.
We run into this same confusion (all the time) with striking a Key on the the KEYBOARD and playing a Key on an external controller. Just because they both can produce a similar result, they are not the same. The MIDI message arrives at the Tone Generator via an entirely different path to accomplish its result.

Those who believe playing an external controller is the same as playing the actual product are basically convincing themselves it is the same... it is ‘similar’ in results, not the same.

For example, if you wanted to use the MONTAGE’s “learn” feature to set the Low Note Limit point for creating a split.... you must do so using the MONTAGE’s physical Keyboard. It addresses the MONTAGE directly, your external controller cannot be used to activate the “Learn” function, because it is not addressing the hardware via the same pathway.

Thanks for your participation, but don’t expect results on this request — in the meantime, just use the MONTAGE to make the MONTAGE assignment, then match that assignment with your external controller. Thanks for the question. (If your thinking it works that way in virtual synths that because they don’t have any direct controllers ...they’re all virtual).