Prophesy is not predicting the future! It is for edification, exhortation and comfort! The Greek word that is translated "prophesy means to speak for another, So "prophesy can mean to speak for God or to be His spokesman.
I have always said it was language that the speaker himself does not know!
You do not add to or take away from the Word of God by manifesting the Holy Spirit as described in 1 Cor. 12-14!
The word says his sheep will know his voice! He does speak to us on a personal level! He does put in our spirit some word of wisdom or knowledge to share with others! None of it takes away or adds to the Word of God!
It is the ESSENCE of adding to the Word of God!
There's what God says in his word PLUS what God says in your "hearts.
Plus.
That's adding.
You are adding to the Word of God.