No, whenever you cast a spell with X in its mana cost WITHOUT paying its mana cost, X = 0, and you can't choose a different value even if you have mana available. Actually, X = 0 in ANY area of the game, except the stack. A Fireball will have a converted mana cost of 1 while in your hand, in your graveyard, in your sideboard, outside of the game and in your library. On the stack, a Fireball will have a converted mana cost of 1+X, with X being what you've paid for it.