The Bible is a very important book. It was written as God’s personal message to mankind. Therefore anyone who tries to add to it or take away from it without direct inspiration from God will go to hell:
I warn everyone who hears the words of the prophecy of this scroll: If anyone adds anything to them, God will add to that person the plagues described in this scroll. 19 And if anyone takes words away from this scroll of prophecy, God will take away from that person any share in the tree of life and in the Holy City, which are described in this scroll. (Revelation 22:18-19 NIV)
This verse comes near the very end of the Book of Revelation. I think it is no coincidence that the Book of Revelation was the last book of the Bible to be written (Probably around 96 AD). Indeed, this passage implies that the Bible at this time was complete, with no more to come at least until Christ returns.
The Word of God is not to be toyed with. Those who have attempted to add to it for their personal gain, like Mohammed and Joseph Smith are no doubt already experiencing horrors that are beyond imagination. This is a very serious sin.