Well I think you can't. If you really want to install some tracks you have to reinstall RSF in to the RBR floder . It will aotomately skip the files you have installed.
if you download & unzip a track say its called "USATRACK" this track has folders like cache, FX ,objects, textures u can drop those into C:\Games\Richard Burns Rally\RX_CONTENT\Tracks\"USATRACK"
and then run RFS go to stages pick the last one TEST BTB STAGES pick your track from the list & ENJOY!