I was given a few packs of grain coffee. The smell was pretty nice and it was written that it's 100% arabica. That encouraged me to grind some and brew it in moka pot.  The result I got was not horrible, but way to sour for me. I tried one bean and indeed it was little sour, compared to Starbucks Espresso Roast, which is not sour at all and has great almost black color.  I am sure this will not be my coffee of choice, but it would be nice to upgrade it somehow (before I throw it away), though I have no idea how to do it. I thought about roasting it in the oven (210-225 Celsius; 410-437 Fahrenheit) for 10 minutes.  Do you think it might be a good idea? 
Sour could be under-roasted, but it could also just be that particular bean variety, or the age of the bean. I would not try re-roasting already-roasted coffee.  What I would do is blend those beans with something to complement them. You can find flavor charts around the internet, such as this one from Coffee Bean Corral.  I'd probably look for something in the "Earthy" and "Chocolatey" realm. Maybe a Brazilian or Colombian. And if you like the roast flavors, look for something that's been roasted to a city+ or so. In addition to the color, oil on the beans is an indicator of a darkly-roasted bean. 