I had a missing sector from a LOT Polish business class flight from Tallinn to Oslo. So I just printed out the missing miles form (no online submission for non SQ group flights), filled it out, scanned it along with copy of my e-ticket and boarding pass, and emailed it to the address provided! They acknowledged the receipt of my info within 24 hours. So that will be the easiest way to do it for non-SQ group flight. Scan and email!
Carfield