am I able to transfer com.sg to Linode DNS?

Linode is not a domain registrar, like, for example (a bad example!) GoDaddy. If you're dealing with transfer codes, then that's what you're looking for.

What Linode does provide is DNS hosting. There's nothing TLD-specific about that – it's not like it involves dealing with the registries -- so it works for anything, barring stupidity on Linode's part. (Plus, I just added example.com.sg as a test and it worked fine.)

Edit: So, no matter what TLD your domain is using, you can use Linode to host the DNS service, but you'll have to find a registrar.