What is required to add a new system call to Minix?
First, there are several system "processes", including the server processes - process manager (pm) and file system process (vfs) as well as the idle, clock, system, and kernel processes.
If you execute the command ps -ef | more, you can see that the last four are listed with negative PID's and pm has pid 0 in Minix.
A user cannot directly make a system call to the kernel or system (except for the message passing function _syscall).
So implementing a new system call available to a user must start with sending a message to a server process.