Equipment hack

Would it be possible to change an item so that it would be useable by a different class, or change the character so it is both lets say knight, and wizard? That way i could use a shield on a wizard because i never use my spell, its useless and i want the fame bonus.

Doubtful. The server should know if an item is for a priest/wizard/etc... and thus easily prevent a hack like this. I've never tried it myself so it might be possible but the odds of it being possible is not very good. Besides, someone would have surely tried this by now. If it did work, we would hear cases of people using other class's equipment.