What you’re going to need to do is look through the code and figure out where the “plugin” or “theme” is actually getting and updating the values. You’ll need to see if there are any hooks their that update the value.
Alternatively, if it updates the post you might be able to add action on the WP post_updated hook https://codex.wordpress.org/Plugin_API/Action_Reference/post_updated. You could use get_post_meta() to get the values that the import creates and use the ACF update_field() function to update the ACF field https://www.advancedcustomfields.com/resources/update_field/.
Your best bet for figuring out exactly where you can hook into the process will probably be from either the theme or the plugin developer.
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