origin and disks work together , Steam versions only work with steam so if i buy the base game on steam i must buy expansion/stuff packs on Steam
here is a guide that will help you http://answers.ea.com/t5/The-Sims-3/FAQ-Steam-VS-Origin-VS-Discs/td-p/3135440
also please take note the layout of the game has change din Origin
http://answers.ea.com/t5/The-Sims-3/FAQ-New-Origin-Layout-for-The-Sims-3-Expansion-Packs/m-p/4641393#U4641393
edit typo
edit edit to add back up your stuff so as not to loose saves etc http://sims3.crinrict.com/en/2011/03/faq-what-files-should-i-backup.html
