Use tee --append or tee -a.
echo 'deb blah ... blah' | sudo tee -a /etc/apt/sources.list
Make sure to avoid quotes inside quotes.
To avoid printing data back to the console, redirect the output to /dev/null.
echo 'deb blah ... blah' | sudo tee -a /etc/apt/sources.list > /dev/null
Remember about the (-a/--append) flag!
Just tee works like > and will overwrite your file. tee -a works like >> and will write at the end of the file.