At the point where your field group is create in PHP and before you call the acf function to register it, json encode it and save it to a file.
$path = dirname(__FILE__).'/'.$group['key'].'.json';
$json = acf_json_encode($group);
$handle = fopen($path);
fwrite($handle, $json, strlen($json));
fclose($handle);
that will save the file in the same folder on the server as the script you put this in. Then import the field group from that file.
