Firstly, apply to a Spore company to see if you are accepted. If you are, then you would probably meet the hours required by CAAS.
CAAS will not issue a Spore ATPL licence if you are not sponsored/employed by a Spore company. You would have to take an Air Law paper and probably 1 or 2 additional papers depending on the strength of your foreign licence.
If it's any consolation, its easier for a non-sporean to get their foreign licence converted than a sporean with a foreign licence.
Currently, SIA accepts Malaysian, Hong Kong and Indian national for ab-initio. These guys do the complete papers.
Good luck