is it possible to credit your miles to different frequent flyer programs for each leg?

for example I'm looking at booking SEA-NRT-SIN through UA, with the NRT-SIN leg on NH, on a 'L' booking class ticket. I would credit everything to SQ's program but 'L' class on NH is no miles on SQ's program but at least gives me 30% on NH.

googling tells me to do a retroactive claim but SQ's retroactive claim process is a massive PITA. can I just ask them to change the FFP number during the layover?

tldr: for SEA-NRT-SIN is it possible to:
SEA-NRT credit miles to SQ
NRT-SIN credit miles to NH