Lets say you would like to add chromium daily builds PPA repository to your sources.list and import its key:
sudo add-apt-repository ppa:chromium-daily
Thats it!!
the repository is stored in :
/etc/apt/sources.list.d/chromium-daily-ppa-karmic.list
the key is automagically added.
Tuxicity
this is only for Ubuntu9.10 (karmic) and above