To devour, grab host with Ctrl + Click or Grab intent (3 in hotkey mode) then click yourself, standstill. To stop the host from getting up and running away, you can switch to an empty hand (X in hotkey mode) and keep tackling them using Disarm intent (2 in hotkey mode) when you are devouring them. Beware the devoured will die in minutes due to the lack of oxygen, being seriously injured and going into pipes will further reduce the time to less than a minute. If the devoured is armed with a knife, they can cut their way out from the inside, gibbing you in the process. (Edit)
Usage
Add {{Devour}} to all xenos, so you don't need to change them one by one if this changes.