Adding melted chocolate to a sponge cake batter
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.