Well, that's going to be an issue, since a standard USB-A plug only puts out 5V. If the old charger puts out 9V at 2A, then that's 18 watts of power it needs, so a 5V USB plug would have to put out at least 3.6A of current. Most computers won't put out that much power from a USB port.
That's beside the issue that I seriously doubt it'll charge correctly on 5 volts, so you'll have to find a way to boost that up to the 9V the light is expecting for charging. All in all really doesn't sound like it's worth it just to open up a little room on the power strip. Be a lot easier to just get a short household extension cord to plug the brick into, then the extension plug will fit just fine in your power strip.