Alpha in transparent material?

[sejnulla]

Hello!
Is there a way, in a material with some kind of transparency, to have alpha in the final rendered image?
Let's say i render a glass material and want to put the final render on top of something and see the background through the glass. Of course the refraction would not be correct but is there a way to do this without having to cheat it in post?
And if it is possible, is it also possible to do it on frosted materials?

Thank you people!