Create Default Value to pull auto populate data from a Look-up Window (Custom Object)

Hi.

Is is possible on our Fullfillment/Lead page to have a workflow or default value created to make the following scenerio in CRM OD work:

When the User selects a Product from the <Product List> look-up window, the stock number identifier is auto populated in the <Product List> field.  This is expected.

Now then, can I have the Product Name autopopulate in the <Product Description> field right below?  The idea is that is would auto populate once the stock number is populated in the <Product List> field.

We have the <Prooduct Description> already associated to the <Product List> identifier in the Look-up window.  Again, I would want this to happen before the entire Fulfillment/Lead is saved.