SQ Krisflyer allows retroactive mileage claim if the flight was prior to enrolment (for SQ flights only):
http://www.singaporeair.com/register...form?showTnC=y

You could credit to another *A carrier program but would firstly need to find the fare bucket they were booked in and whether that would earn any miles in that program, then check whether that program actually allows retro credit prior to enrolment for an SQ flight.

For example, Aegean provides no credit for SQ flights booked in O, I, X, N, G, Q, T, V. They credit 100% for all other economy fare buckets, and a MEL-SIN-Europe return would likely earn enough for Aegean Gold (*A Gold).