Rockbox will do nothing to your existing music / playlists / other random content Apple has left strewn on your iPod. It leaves them all alone.
In order to access your current selection of music from Rockbox, you need to read the manual - specifically the section on the Database.
No more questions until you've read it. If you still have questions *after* that, then please let us know which section of the manual you're having problems with.