With scripting, it's possible to do. You can even pop the list up in a small floating window where you click a product record to add it to your portal.
You can also manage larger value lists if you break them up into categories and set up a conditional value list so that you select a category in one value list formatted field and then the ProductID field's value list lists only products that are a member of that category.
Here's a demo file with conditional value lists: http://www.4shared.com/file/f8NsU2DJ/ConditionalValueListDemo.html
You can also, with scripting set up the drop down list to be an auto-complete equipped list of product names that is yet able to enter the Product ID into the appropriate field.
Here's a demo file that contains that method (among others): http://www.4shared.com/file/plr_jbkk/EnhancedValueSelection.html
If you'd prefer to use the exact method you describe. Let me know and please let me know whether or not you've done any Filemaker scripting before.
Thx Phil, will check them out, and yeah i never done any scripting before :). Anyhow i let u know my results, thx again for fast respons and help