I have a long layover in Singapore (20 hours) and thinking of getting my baggage checked throught to my final destination. However, I am planning to do some shopping while in Singapore. Am I allowed to check in additional bags at Singapore airport? Or since I'm considered a transit passenger, am I not allowed to check in additional bags? Thanks.

Edit: also where can I insert our non sq frequent flyer on sq flight redeemed through krisflyer? I can't find the menu during booking / check in. Thanks