Yes; all currencies are converted into USD to be invested as USD.

SGD to USD conversion is done at the spot rate and the only cost associated is the currency conversion fee, which is charged by the broker, at 0.1% of the transaction amount. This preferential rate, which StashAway has negotiated with the broker, compares favorably against the significantly higher currency conversion fees charged by commercial / retail banks (easily between 0.35% and 0.70% depending on transaction size).