How does not practice freebsd kernel programming?

[Michael M. Press]

You can play with adding system calls to FreeBSD. This is
not something that people need to do very often, but it is a
good way to give yourself some experience writing code that
runs in kernel mode. The following OnLamp tutorial is intended
for OpenBSD, but I think most of it applies to FreeBSD as
well:

http://www.onlamp.com/pub/a/bsd/2003/10/09/adding_system_calls.html