I am planning to use my UA miles to fly from any of the three New York airports to Singapore in SIA C.

Assuming I can get the seats, is it possible for me to fly non-stop from EWR to SIN in SIA C and on my way back fly SIN-FRA-JFK with stopover in FRA? Also, will they charge me 90K or will they charge more since I want SIA?

Thanks for the response!