No.  Parts 9-16 cannot have keyboard control turned on so you cannot group them together like you can Parts 1-8.

You have to target each Part 9-16 individually using its MIDI channel number which always corresponds to its Part Number.   At least for every mode other than Single Channel MIDI mode which I would not recommend using.

You can set up Parts 1-8 with Zone control turned on (both Part and Master).  Then in Part 1 set the MIDI output channel to Part 9, Part 2 output to channel 10 ... and so on.   Then place a MIDI cable.between MIDI OUT and MIDI IN (so called MIDI loop back cable) which will allow for Montage itself to play channels 9-16.  There's some flexibility lost doing this but it does work for "most" cases (YMMV).

Or, if you're OK with having a computer or tablet attached you can use the external device to route incoming MIDI back out to channels 9-16.  This would be similar but would not require setting up zones or changing the MIDI output channels per Part.

Or,.you could find an external MIDI controller that allows 16 zones so you can use it to target all 16 Parts.  If you just want to target Parts 9-16 then you can find one with 8 zones.  A Novation controller I have is 8 zones.  Native instruments controller I have is 16 zone.  Many Kurzwiel synths are 16 zones.

Not being able to turn keyboard control on for Parts 9-16 is a limitation you have to somehow work around.