You do it the same way as with Ubuntu, but the repos must be for Debian and
must also be for the same level, i.e. don't mix sid (unstable) with lenny
(stable) or squeeze (testing).

A typical line might look like this: deb
ftp://sunsite.cnlab-switch.ch/mirror/debian/ stable main contrib non-free

You cut and paste lines beginning with deb like the one above into Synaptic.