You can enable the source code repository and download/install the kernel source code with the yum command. Alternatively, you can download the kernel source code RPM file from a CentOS mirror site and install it directly, again with either yum or rpm commands. And yes, you will need the kernel source if you are going to add your own system call. I would strongly suggest that you do some studying of how to configure, build, and install custom kernels. There are a lot of documentation, manuals, etc on both the kernel.org and tldp.org (The Linux Documentation Project) web sites that will help a lot.