Are you using Cinerender?
if yes then here is how you do it.
The place where the options are might vary depending on which version of Archicad you are using, BTW please add your signature with your Archicad version to receive more accurate answers to your questions.
the 1st step is optional, only if you want to see the render without the sky in Archicad, but not neccesary at all.
The 2nd and 3rd are the key here, this will give you a png with a transparent background that will let you compose the render in any photo editor software.