Can I add melted chocolate to my already made vanilla buttercream to turn it to chocolate buttercream?

Sure! I take most of my recipes like that and play and come out with some amazing results. Make sure that the chocolate is warm and not too hot, otherwise it will melt your buttercream.
You can add cocoa powder to it or even chocolate syrup is good too !

If adding liquid chocolate, you might have to add some more p.sugar to balance ... if adding cocoa (powder) you might have to add more milk/water to balance that.