I'm not sure what exactly "a battery back up system like from best buy" is, but we have on occasion used battery backups that are not integral to the fixture. This tends to happen when the customer or Architect chooses a fixture that doesn't come with a backup battery option.
As iwire mentioned, the battery backup has to be listed for emergency use. I know we've specified models from Philips and Lightalarms in the past. I'm sure there are others.
If your Best Buy battery backup is a plug-in UPS like you would use for your computer, I doubt it has the correct listing. Plus, can lights are generally built to be hard-wired and your Inspector may not like it if you attach a cord and plug instead.