Back in the early to mid 1990's, SIA used to hire F/O's direct from GA in western countries (eg, Australia, USA) but probably due to a few of them taking SIA to court for whatever reason (it has been the subject of numerous threads and posts on PPRune), they stopped doing it.
Now, SIA trains their own F/O's at their own flying academy in Seletar and Perth, Australia. They recruit Singaporeans as well as citizens from Asian countries for this, namely Malaysia, India and Hong Kong, but never any western countries, probably (as a guess) due to the events mentioned above.