Need 18 channels for a VTOL in Mavlink interface but only 16 channels are supported, Using px4 sitl with gazebo

Hi dear px4 developers!

I am using px4 sitl and gazebo to simulate a vtol drone and to tune the control. This drone has 8 tilting rotors, 2 flaps, elevator, and rudder. I used the tiltrotor files to develop our custom model.

I was able to run the simulation and observed that the drone has a good performance in multicopter mode but it cannot fly in the fixed-wing mode and it falls immediately after transition to fixed-wing!

There are 18 channels (by order, 8 for rotors, 8 for tilting servos, and 3 last ones for control surfaces) in the MAVlink interface of the .sdf file for this drone but I figured out that the reason for fall is because the last three channels in the sdf file cannot be read by the MAVlink so they are not transmitted to the px4 sitl.
I also figured out that there are only 16 channels available for the MAVlink.

Could you please help on how I can have all the 18 channels to be read in the model?
Or how can I use just one channel in the sitl for all tilting servos?

I highly appreciate any comment or advice