For the pouch, I've had the best luck taking an extra string (make it looong) and looping it around the strands from the braid -- divide them into two groups, then four, then eight, until you get the right width.  For adding strands, just thread them through the middle of the braid, 2-3 rows up from where you left of, and let them hang down, half on each side.  Then braid them in by doubling (quadrupling, whatever) up on each 'element' of the braid.  A better solution is to add extra strands to the core of a round braid with core, but that's a bit trickier.

Um, kind of...  I know a picture would explain it better, but I'm graphics impaired right now.  Let me see if I can make it clearer.

You're doing a four-strand round braid, right?  Basically, two strands cross over,  then the other two strands cross, then the first two, then the second two, etc. -- unless you're using the over 2/under one method.  Either way, when you look at the braid, you see two threads crossing back and forth across each other, and two more threads doing the same thing, but going in and out of the loops created by the first two threads.  Hope that makes sense.  Just take your extra cordage and poke it through one of those loops -- right through the middle of your braid.  Then pull the cord(s) through, so that the center of the new cord is the part going through the braid.  Then let it hang down with the 4 elements you're already working with -- you've just added two more elements (if you stuck one cord through).  Then you can double up with the cords you're already using and they become part of the braid.

Sorry, I don't have a digital camera, & I'm no good at .jpgs.  Good luck!