To edit the commentary by hand, as it were, you need to go to the Match Events file - C

Program Files\Sports Interactive\Football Manager 2006\data\match events.
Once there, open the 'events.CFG' file with Notepad and here you'll see all the commentary files used in the game. Just replace the text here with your own and save it. For example, you could change the line "He looks happy with that one" to something like "That's put the fizz in his bottle!"
No need to start a new game after editing either so you can see how your commentary fits the action as you go along.
Have fun!