I do this all the time, but is not productive.
The most productive way is to build the library automatically using the makelibrary
command. Use the Tools->Schedule automatic library update menu command to create an event with this command. You can also make it process tracks: trim silence, detect mix points, scan for BPM etc.
Manually, you can do it like this:
- clear the library: Ctrl+A, Delete
- add folder
- Tools->Process tracks (if needed).