ACF saves the custom field and its reference key in the database. The custom field entry is like this:
custom_field_name => the value
While the reference key entry is like this:
_custom_field_name => field_1234567890abc
Where ‘field_1234567890abc’ is the field key. If you want to add it manually, you need to add both of them to the database. If you want to add it programmatically, you can use the update_field() function instead.
Also, it seems that WP All Import has an add-on for ACF. You can check it out here: http://www.wpallimport.com/advanced-custom-fields/. Unfortunately, I’m not so sure if it will work or not. Please ask them first if you want to use it.
I hope this helps 🙂
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