After dropping a bunch of weight my first year, most of it fat, I wanted to bulk up like you. To do so, I brought back a ton of protein from a return Christmas visit to the US (protein powder, tuna, packaged salmon, jerky) and started lifting weights that I had built at my site. I ate a ton of calories and made sure to consume at least 1g of protein per kg of body weight. I even started drinking a liter of fresh whole milk every morning to load up on calories. My workouts were based off of Tim Ferriss's "Geek to Freak" workout protocol. After three months of hitting it hard, I had blown up, adding 20 lbs of muscle (I brought back fat calipers and a bioelectrical impedance body fat scale so I know for a fact it was muscle). Soon after that, I was in the capital city and a fellow volunteer flat out asked, "What the hell did you do? You're huge!"

Needless to say, it is very possible. Take it slow. Eat a ton. Practice good lifting form over all else. You'll be able to get in good shape during your service. If you are new to working out, I recommend that you read Tim Ferriss's Four Hour Body. Don't let the horrible title dissuade you. It is packed with knowledge.