I have never tried this, but my Update Manager's Software Sources has a line (not checked on my system) "file:///usr/share/local-repository" in the Other Software tab. I assume that you could download the files manually, place them into that directory, and enable that directory as a repository.
Hmm. Not sure how you would get your computer to realize that there is an "upgrade," though. Maybe do it through Synaptic (or a terminal) once you have grabbed the files and enabled your local repo?