Standtbahn Di Berlino Per Train Simulator.

This will not happen.
If a Publisher wants to have their product on Steam they approach DTG and if it passes DTG Publish it.
A DLC sold on Steam will generate less revenue for the publisher which is why it won't happen.

But because you seem to think McAfee is telling you the truth you are missing out.

How does what you have bought on Steam get on to your computer - everyone uses the Internet to connect to Steam.
Or do you have some form of magic to make that happen?

 

Steam runs on ….. steam of course. It is pre-internet.