The Chance cards are defined in snippets (eg career_Business_ChanceCard_13) listed in the super interactions in basic_extras (like in si_Career_Business), or directly in the basic_extras (like in si_Career_TechGuru). I think Business and Atheletic have a different structure because they were made afterward.
To make a new Chance Card, you could add it directly in the basic_extras, or create a new snippet that you add to basic_extras (take a look at the document to generate your new ids : http://forums.thesims.com/en_US/discussion/779844/maxis-documentation).