How to join audio channels in matlab ?

I am thinking of creating a small utility to convert (upmix) a stereo based mp3 to 5.1 channels. I was wondering if someone could provide me some info about it. Initially, i started with reading the mp3 files, converting them to a double matrix with 2 columns. The first column corresponded to the right signal and second column to the left signal. On separating them, I obtained two mono signals. Now, I want to do some type of signal processing in this matrix and then distribute different types of sounds in different channels (possibly 5/6 channels).

The problem is that I know how it can be done theoretically as I have learnt it in my courses, but practically it is getting a bit over my head. A little nudge in the right direction will be helpful. Thank you.