I have a Canon EOS 1100D and I would like to shoot a long video, of a long concert, for example. Is there a way I can charge it while it is taking a video (not just when the battery is in the camera like this question asks)?
What you are looking for is the "Canon ACK-E10 AC Adapter Kit" which does not charge your camera but instead allows it to run off of AC power.
But keep in mind that the 1100D/Rebel T3 is only going to allow you to record 17 to 29 minutes of video at a time. You will have to stop and start a new recording every 17 to 29 minutes depending on the settings you use for the video.