batch processing - converting and adding AAC tracks

Is it possible to write a bash script (maybe using something like ffmpeg) to detect if a movie:

• contains no AAC 5.1 audio track
• does contain an AC3 (5.1 or higher) audio track

and convert the AC3 to AAC, while keeping the AC3 track as-is, and adding the AAC as a secondary track to the movie?

From what I've been able to find no such tools exist to do this in batch-processing, and since I have a lot of movie files it'd be too much work to do this manually.

I'd love to hear some thoughts, pointers, examples or idea's about this.