no, this functionality is not available. there is no 'hold' button on the iPhone, and as the other poster discussed, you can change the volume while playing music and the screen is locked, but no other time. 99% of the users would find that the iPhone would be incredibly annoying to use if you had to take it out of your pocket just to change the volume levels.
can you adjust your arm band so it is not so close to the volume level controls?
karenvb, i have a Sena case for my iPhone, and i have never accidentally changed the controls while walking, working out, etc. i also grab my iPhone by the front and back when taking it out, so i don't hit the volume controls at all.