Actually I believe it's more like

  dpkg -i -E -G -B --force-overwrite --force-breaks --force-conflicts --force-depends --force-depends-version *.deb

with one or two extra things, like installpkg/upgradepkg will prompt you for what you want to do with config files, and adds one or two extra heuristics. But, yes, dpkg could totally be used in a similar way as upgradepkg. It's just more complicated so it doesn't work as well ;)