@burcidi- Yes, citronella grass can be used in cooking since it is a herb and in the same family as lemongrass. In fact, when I was in South Asia last year, lemongrass and citronella grass were used interchangeably. Asians like to use this to flavor meats, seafood and soups for the most part. It was also a popular ingredient in tea along with ginger.
I don't know why but lemongrass is usually preferred over citronella grass in cooking in the West. When we think of citronella in the U.S., we first think of insect repellent, aromatherapy, soap and candles; not food for some reason.
But you should definitely go for that chicken and citronella grass soup! I think it will be delicious!