convert osc2nuke to joomla

I have a website running on osc2nuke. It is a integration of oscommerce on php-nuke. The database has 4500 users, 600 files on the download area, a integrated phpbb forum with 2300 posts. On the store i have about 1000 items. There are some other modules but they are not important. I want to change to joomla 1.5 with virtuemart Need help doing the transfer of the database. I accept suggestion on the jooomla extennions to use to manage the forum and the downloads. Transfer of the osc is not necessary.