Is it possible to Pause the running upgrade of a building?

No unfortunately not. You can only speed it up.

I believe that you can delay the completion though by closing the app and not being in the game until you want the build to complete. This used to be the case anyway. If you are not in the game then the build will not register as completing until you log in again. So if you need to delay the completion for a few hours to get points in an event you can try this.