If you use CSM you just have to install the mods with self-installer and load them( with CSM. Some mods are real masterpieces and are worth a try, for example F1 1991 or DTM 2005 and 2006 available on www.grandprixx.com and www.gp4db.com.
If you want to have more recent cars (such as F1 2010) you have to wait for the cars to be released and install the addons as long as they are released... just be patient. It's not difficult to install them with Easywad (which comes with CSM) but takes a bit of time and you have to remove the old .gp4 and .tex files in the wads and to rename the .gp4 files in order to have the correct pitlane order. It takes a bit to understand how it works but once done you'll be very satisfied and will forget about the default 2001 cars!