You cannot. Maemo works by allowing the package management system on the device to handle all software installation and uninstallation. What you can do is download software packages using your desktop and transfer them to the N900, which will install them.

However, this does require a bit more knowledge of how Linux works and is a non-standard process for Maemo, which could introduce problems or be difficult if you are unfamiliar with the process. If your problem is that you don't have network access, you may want to look into using internet connection sharing via an ad-hoc wifi connection or USB networking.