Re: nvidia geforce
DesktopBSD may or may not provide a tool to automate fetching it (I've never used DesktopBSD; only Open, Free, Net, and PC); but you can always grab it in the usual way.
You can create a supfile for use with csup (or cvsup), or just use command line arguments to fill in the gaps. You should look in /usr/share/examples/cvsup for some example sup files. The FreeBSD handbook gives a detailed overview, and you should also find several quick examples in the PC-BSD forums, DesktopBSD forums, and PC-BSD knowledge database.
The basic idea is, you ask the program to fetch source code for a specific branch. This populates /usr/src/ with the source code for that branch (which should be the code for your running system; unless you want to do big updating).
The nvidia drivers build a loadable kernel module (LKM) as a requirement of being a driver; so it needs the kernel source code to build correct; and for maximum stability, the kernel, the userland, the driver, and the FreeBSD source code should all "match up" in versions.