apt-get grabs the required .deb packaged software form the repository and automatically fetches and installs any dependencies you may lack.

What you would really need to do in order to get some software that is instaled on someone else's N900 is not only tar up or zip up the binary for the application (usually under /usr/bin) but you would need to compress ALL the files originally distributed with the original install's .deb file.

This can include library files in /usr/lib/*, other binaries, optional data files, etc.... It can be anything and without knowing what .deb you are trying to get, we can't be more specific.

Once you have all of these, they would of course need to be unpacked to the same directories on your N900.

You would also need to identify and install any dependencies that you may lack. This is harder then it sounds because if you are missing anything, the only symptom that you'll observe is the game failing to run.

If you're installing from a .deb with dpkg, application manager, or apt-get you will clearly be told what additional packages you need.