Is there any possible way to install steam from a .deb file to a folder on an external usb hard drive? I have tried something like "--root=" but I can't get it to work.
Installing a package to a different location isn't supported, as a package is usually carefully constructed so that every file is placed in the correct location, and having those moved elsewhere at the whim of the user means that the appropriate files can't be found anymore. The packaging system also doesn't cater for different places for the files, so removing the package won't be possible.
If you just want to extract the package somewhere, then that's not a problem:
dpkg-deb --extract whatever.deb /some/path
If you also want the control information (file list, pre- and post-installation scripts, etc.) to be extracted, use:
dpkg-deb --raw-extract whatever.deb /some/path
The control information is placed in /some/path/DEBIAN/.