I see in the SharePoint Designer that you can specify a list of Association Columns that will be automatically added to a list when a workflow is associated with the list (if the columns don't already exist). What I can't figure out is how to add the list of Association Columns to a workflow definition when you design it in Visual Studio.
You can create workflow association from. On this form you can use controls that should contain information about needed fields. This information should be stored in association data. You can retrieve this data in your workflow and add needed fields to your list through code.