Hi Sephiroth0327,
In SharePoint, you can create business data column in your list. Then in your SharePoint designer, you can use the
column. You can refer to the following link.
http://blah.winsmarts.com/2007-4-SharePoint_2007__BDC_-_Using_BDC_Data_in_Lists.aspx
But I suggest you use Using SharePoint workflows with Business Connectivity Services (BCS) – Sandboxed Workflow Actions.
This is support external list.
You can refer to the MSDN blog.
http://blogs.msdn.com/b/bcs/archive/2010/01/29/using-sharepoint-workflows-with-business-connectivity-services-bcs-sandboxed-workflow-actions.aspx
Thanks,
Jack