Adding Fields tp Contenttypes programatically

The SPContentType object also has a Fields property that returns an SPFieldCollection object. You cannot add columns directly to this collection. When you add an SPFieldLink object to the FieldLinks collection, a corresponding SPField object is added automatically to the Fieldscollection. Each SPField object in this collection represents a "merged view" of the base column definition and any overwritten properties that are specified in the column reference.


http://msdn.microsoft.com/en-us/library/aa543526.aspx