I took the challenge. I have a Z5, FTZ-II, and a very cheap 2x F-teleconverter I inherited. Keep in mind this is a (dumb) full manual teleconverter and has no AI connector pins for the lens. So no ability to control f-stop here on my 70-200mm lens as there is no aperture control ring on it. But it did take an actual photo and did not return any error.
So it
can work. It
might work with another teleconverter that has AI pins.
Will autofocus function right? Unknown.
Will it work with autoexposure? Unknown.
Would anybody guarantee their teleconverter to work? No.
Try it and hope.
Should you use the 18-300mm lens in this combination? I say No!! Even if it functions. This is a DX lens and will only use the center portion of the Z5 24MP sensor. You will now be reduced to 12MP resolution, get all the optical degradation of the teleconverter, and the 3-stop reduction in exposure value that comes as the penalty of using a 2x teleconverter. You are stacking 3 bad things into your images and will be disappointed.
And another thing to know is that the Nikon Z 2x teleconverter is only compatible with certain Z lenses. You have to research which ones are compatible to see if it is a good fit for you before buying anyhow.