I have baked a victoria sponge cake and it came out fine. In order to make a chocolate sponge cake, Can I use the same recipe of Victoria sponge cake and just add some quantity of melted chocolate to the batter? Will that work?
Will it end up being a (probably delicious) cake? Yes. Will it have the texture of a Victoria sponge? No. The melted chocolate will destroy some of the airiness of the sponge, making it more like a brownie.
Instead, I would recommend substituting a few tablespoons of flour for unsweetened cocoa powder. That will give you a chocolate Victoria sponge.
Egg whites are generally whipped without the yolks to create more volume. There is fat in the yolk which keeps a good "foam" from developing. Chocolate would have some fat too. Not knowing the ingredients for your recipe, nor how the ingredients are used I don't think a definitive answer exists. So the answer is to try it and see what happens.