Can I use an Apple Homepod as a speaker and microphone for WebEx, Skype voice, etc?
Unfortunately no. It is not possible to use the HomePod's built-in microphone for recording audio to a Mac. You cannot Bluetooth pair the HomePod to a Mac in order to use it as a combined speaker/microphone either.

Other products exists that does what you're looking for. For example I'm using the Logitech Group speaker/microphone combination for web conferences. However, it is quite a different product from the HomePod.