It IS the roast that is the difference. The only real difference in the beans is that some beans taste better at a higher roast than others, so they are more appropriate for espresso. Your Italian grocery coffee company may be using the espresso label for marketing purposes, but in general, espresso coffee beans can be the same beans that are used for "regular" coffee, but roasted to a French or Italian roast level, which is darker than City or Full City.
Since the advent of Starbucks, many roasts are much darker than they used to be. Dunkin' Donuts coffee, which is a Full City roast, used to be the norm, but now a French seems to be what you can buy.
I roast my own coffee and take it to just into the second crack which is, generally, a Full City roast...a point where the character of the coffee predominates rather than the flavor of the roast. There is more information about roasts at Sweet Marias where I buy my green beans, and reading through the site will give you way more of a coffee education than you probably ever wanted.
So, yes, you can use the coffee you have to make brewed coffee. It will probably be roastier than you would normally have, unless it is just a marketing ploy, in which case it will taste normal. Consider how long you have had this coffee; if it has been shelved for a while "normal" probably won't be all that great, since freshly roasted coffee is, generally, way better than old coffee. But as long as the oils aren't rancid, it is more likely just going to be bland.
It has nothing to do with the microwave and everything to do with the volatility of aromas and flavors in coffee. Even coffee kept warm for 4 hours won't taste very good. In my experience, stale brewed coffee results in a more pronounced acidity, if left out to cool, or a woody, muddy, bitter kind of flavor, sometimes with more pronounced acidity, if held warm. If you were to make a straight espresso, it's optimal to drink it within 20-30 seconds. Brewed coffee can usually be held warm around 30 minutes before pronounced deterioration becomes obvious.
A popular Japanese TV show suggested perhaps stirring a pinch of salt in to stale, reheated coffee, which may trick your tastebuds enough into tolerating it. I've found that tactic less than adequate, but your mileage may vary.
There's a ton of information on coffee flavor compounds and other coffee-related research at: http://www.coffeeresearch.org/science/aromamain.htm
Best Answer
Yes, you can.
In fact, this is a specific technique known as a "coffee toddy" which is a good way to take liquid coffee camping with you if you don't want to take brewing equipment. Some people even prefer coffee made this way to other methods of brewing.