i think you're re-heating it wrong. put it in a bowl, add a smidge of water to it (or milk, depending on what the cheese called for)...i am talking a minuscule amount. then you take a paper towel and get it wet, and place it over the top of the bowl so it acts as a wet lid. this will keep the moisture retained almost entirely. then, set your microwave to only heat at like 20-30% for about a minute. just so you can get some warmth back into the fake cheese and get the juices flowing again. take the bowl out, stir a bit (which will stir any remaining water at the bottom that you added back in at this point), put the paper towel back on, and give it about 70% power for another minute. by now some heat should be getting into the macaroni pretty well, stir again, re-wet the paper towel if it needs it, and then put it back in at 100% for about 30 seconds. you dont want to go too long on 100%, it might not even be necessary. but by then, you should have rejuvenated the macaroni to about 90% of what it had been when you first cooked it. from here, add what ever you want to it....canned tuna works really well and is super cheap.
I wish I could still eat macaroni and cheese, it was the hardest thing to give up that delicious velveeta shells and fake ass cheese....damn you health problems!