Answer by easement for How can I theme the template for edit or add a node for a specific content type?

Hmm. There may be a better way but what about a preprocess function.

I'm still really new to Drupal, so I would maybe try something like this [code may not work]:

<?phpfunction themeName_preprocess_page(&$vars, $hook) {  if ((arg(0) == 'node') && (arg(1) == 'add'&& arg(2) == 'product')) {    $vars['template_files'][] =  'page-node-add-product';  }}?>

Be sure to clear cache and theme registry after making new preprocess functions.