Microsoft doesn't make the specs for the interface between the Windows kernel and the win32 libraries public.

Nor it is been stable. Win8 for example changes this interface.

Even the Windows API `system calls', like CreateProcess, are just wrappers around NT system calls. There are other wrappers too, like the Subsystem for UNIX Applications (which implements a POSIX API on top of the same NT calls).

You cannot call syscalls directly, as the syscall table is generated dynamically from the kernel source code on NT. The closest thing you can do is call the Nt* class of functions, which are often direct thunks to syscalls.