Yes and no. You can add cocoa powder to vanilla buttercream, but since it is dry and powdered, you will also need to add liquid. Depending on how much cocoa powder you add, you may also need to add more sweetener since the cocoa powder is unsweetened. Additionally, it may be hard to get the cocoa powder to disperse evenly in the vanilla buttercream without clumping together, so you'll want to add it slowly while stirring constantly.