Adding extra audio tracks to an mp4 movie file?

I'm new to using iMovie or similar programs for converting/merging video and audio, so any help that steers me in the right direction is greatly appreciated.

I made an mp4 file of a movie, and want to add an audio track to the mp4 file. The audio track is an mp3 file. However, in order for me to have the new audio track sync up with the video, I need the audio track to start playing before the movie begins. Please note that I'm not interested in burning this to a DVD; I just want a new mp4 file with the extra audio track.

With this in mind:

1) What program would be best for merging an mp3 file with an mp4 video file, plus adding extra time to the video in order to accommodate the extra time in the audio track? I have iMovie HD 6.0.3, and the latest version of Handbrake, unless another program is better for this task.

2) What steps do I need to perform to merge the two files? I'm having trouble figuring out how to do this in Handbrake, and importing the mp4 file in iMovie will take forever.

As I said, I'm new to using these programs, so if you need me to clarify anything, please ask and I'll do my best to help out. Thanks!