Anything is possible. You could modify the feeder to just add this to every one of your files if you like, however, the product attributes mod is very helpful for the products you have that may offer UPC codes (google likes having this info anyways, so it's a good way to go).
This is in no way tested, but, it 'should' work.
Find the line:
$item->appendChild($dom->createElement('grice', number_format($price, 2, '.', '')));
in includes/classes/google_base.php
then after that line, add the following:
$identifier_exists = $dom->createElement('g:identifier_exists');
$identifier_exists->appendChild($dom->createCDATASection($this->google_base_xml_sanitizer('False')));
$item->appendChild($identifier_exists);
save the file, and this 'should' make sure that every item you export has the identifier_exists attribute.
Please note, there is no way that I am aware of to make this happen without editing the module files. This isn't a core file edit, but specifically just one file in the feeder module.
You will need to have the ability to access and edit files in your zen-cart install. Additionally, I can not guarantee this edit will perform as expected as I do not currently have a non-production cart for testing, so, backup before you edit!
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