There was a conversion kit available for the mossberg 500 but it is no longer produced. The only way that I know to get one now is to find one used. I have been looking for one for myself for a while now.
This is one of my favorite mossbergs to date. I wish the mossberg bullpup was not so hard to find. If anyone has one that they would like to sell please let me know. I would even be interested in a conversion kit if someone had one.