One very useful feature in SharePoint Designer (SPD) Workflows is the ability to interact with External Lists, thanks to the new Business Connectivity Services. This primarily allows you to create and update items through workflow actions. You will need to make sure that your External List’s Content Type has at least Get Item and Create Item methods defined. Let’s go through how to build a workflow action that creates a new item in an External List.
1. Open up SharePoint Designer 2010 and connect to your site.
2. Click “Workflows” on the side menu 
3. Let’s create a List Workflow, so select “List Workflow” on the ribbon and choose the list that this workflow will associate with. 
4. Give the workflow a name and description then click Ok. 
5. Click inside “Step 1” and click the “Action” button on the ribbon. Choose “Create List Item” 
6. A new workflow action will have been created inside “Step 1”. Click the hyperlink “this list” to choose the external list to create the new item in. 
7. When the dialog opens, choose your External List in the “List” drop down. Click the “Add…” button and start mapping fields from your External List to workflow properties or variables. 
8. Click OK when you’re done. Step 1 in the workflow should look like the below image 
9. Add any additional actions to your workflow then save and publish it to the list. 
10. Go back to your SharePoint list that’s associated to workflow we just created. Create a new item and manually trigger the workflow. When the Create Item action is executed, a new entry should be created in your External List.
Additional References:
Using SharePoint workflows with Business Connectivity Services (BCS) by JD Klaka
Peter Sullivan - SharePoint, TFS and .NET Learnings 