The heterogeneous rigs are tricky, but still possible.  A good approach is to combine two different presets:

Example: Using zCam for main cameras + GoPro for nadir or sky

 First, stitch the zCam footage using the zCam preset. 

Then, poach camera settings for the GoPro from a GoPro based preset (the Omni for example): Apply the Omni preset, (it will get completely broken), see the "Camera Defaults" tab and write down the relevant values: Focal length (16.449) lens distortion (a,b,c = 0.0, 0.041, 0.0) and circle mask (100, stock GoPros are full frame). 

Now, undo to get your zCam stitch back, and add the GopRo camera to the stack. Select it, and in the Input Camera tab, fill the above data only for this camera. (Do not fill this in the data in the Camera Defaults tab, as it would affect all cameras, which we do not want. 

Set the Pitch to 90 ( additional gopro is pointing up), and increase the roll until you find an approximate match with the rest of the cameras. 

When the camera seems to match, use the "Improve Angles" to refine the added camera heading.