In PF18 if you already are in season 5 but you want to start the league over, can you and if so how?

You have to export and import into a new league. Any changes, logos, etc. you made to the teams will have to be redone though. And you'll want to make sure that the team id's match up... so if you've done expansion/contraction and the game has generated new team id's you'll probably have some players come into free agency. (You'd have to mod the teams.xml file to update the team id's to whatever your new team id's are)