You need to check with CAAS on whether they will allow you to convert your FAA licence after writing all 14 CAAS ATPL papers. I don't think there's a need to convert your FAA to EASA then to CAAS. If they allow, you just need to register for writing the CAAS papers. Whether you need to sign up for ATPL Ground School depends on how much help you need. If you think you're ok with your theoretical knowledge, then perhaps what you need to do is to download the CAAS ATPL Learning Objectives (
http://www.caas.gov.sg/caasWeb2010/e...for_Pilots.pdf), get a set of books (CAAS recommends CAE Oxford Aviation Academy ATPL Books), and study on your own. If you need further help then you can contact either SFC or STAA for their ATPL courses.
Don't jump into it until you get something concrete from CAAS on what they want you to do to have your FAA converted.