Can you attach a Nikon P900 to a telescope to get more optical zoom?

Although I'm not familiar with the Nikon P900 system, adding a camera to a telescope is usually an enhancement. The only downside would be if you are trying to exceed the usable magnification of the overall system. Despite manufacturers claims of high magnification (for sales purposes), no telescope is much good above 100 power, and Excellent images are usually achieved around 60x. High magnification means magnifying the atmospheric and optical defects as well as the image you want.