Have you tried modifying the bibliography templates? Go to Output Styles, Edit [Style name], click on Bibliography, then Templates. Set your cursor into the template where you want the URL to appear. Click on Insert Field (upper right-hand corner), then click on URL to add the URL field to the template for the articles. Save, and check that it worked by previewing one of the articles you want this to show in. If you later want to print a bib without the URL field, simply go back into templates and remove the URL field.