How to add Ubuntu repositories on Kubuntu?

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http://askubuntu.com – I have noticed that Kubuntu (I am using 12.04 version) doesn't use exactly the same repositories that Ubuntu. Or at least, that is what I think. For example, if I search for Steam on Muon Software Center or Muon Package Manager, I just can't find it; even if I use sudo apt-get install steam-launcher. But on Ubuntu I have no problem. Why? That's why I think Ubuntu has some repositories that Kubuntu doesn't. In this case (I don't know if I'm right), how can I add this Ubuntu repositories? Steam is only an example. There are many packages that I can't find on Kubuntu. Another example: Motorbike (HowTos)