In my application i need to do something similar. I have a custom Task List with custom fields and associate the Workflow with this Task List.
Now Inside my workflow code when the Task Completes it triggers my task1CompleteInkove method and inside there i have a reference to a property of type SPWorkflowTaskProperties and access my custom fields as so
private Guid CustomCommentsColumnGuidId = new Guid("5AE36B4C-3996-4C25-A13B-FF22A7557773");
string taskcomments = wftp.ExtendedProperties[CustomCommentsColumnGuidId].ToString();
CustomCommentsColumnGuidId is a private GUID field set to the GUID of my custom field in my custom Task List.
Hope this helps.