No. KrisFlyer membership details cannot be added after a purchase has been completed.
This applies to:
Online purchases made on KrisShop.com
Inflight purchases made on Singapore Airlines or Scoot
Why can’t KrisFlyer details be added after purchase?
For security and verification reasons, KrisFlyer mileage accrual requires the account to be verified at the point of purchase.
As a result, post-purchase changes to add KrisFlyer details are not permitted.
What should I do instead?
For inflight purchases
Provide your KrisFlyer membership details to the cabin crew at the time of purchase, during the flight.
For online purchases
Log in to your KrisFlyer account before checkout to ensure your membership details are captured correctly.