How can I import some columns only if the destination column contains an empty value and has been never set. The problem happens when I set the news_to_date. If in the value replacer I set something like this:
news_to_date: {{date('Y-m-d H:i:s', strtotime('+7 days'))}}
The destination on every magmi product update, is set to the current day + 7 days starting from today, but I need to keep the original value if the news_to_date attribute is already set.