The Library tab is used for channel strip settings, instrument and plug-in presets, and MIDI assignments (in the case of MIDI tracks). You are talking about audio files. That doesn't inherently involve the Library tab.
You are talking about importing mp3 files into Logic. The piano roll is for editing MIDI notes. If you want to work with your samples in the Piano Roll Editor as MIDI notes, you will need to create EXS 24 instruments that contain those samples first.
But to get back to your original question, there are several ways you can import with mp3 files into Logic. You mention not wanting to bring them in one at a time. Does that mean you want to batch convert them to aiff or wav files?
If so, use the Audio Bin. You can import each of the entire folders at one time. Logic will convert them to aiff or wav files at this stage. You will then have two versions of each file, one mp3 version, and one aiff or wav version. You can then delete the mp3 files in the Finder if you want to.
If you want to, you can import the mp3 files directly onto Arrange Window tracks and have them remain as mp3 files. I wouldn't suggest doing this though.
You can also use the File Browser tab (in the Media areas of the Arrange Window) to navigate to the mp3 files in the Finder and import them from there.
But I have a feeling this isn't getting to the heart of what you really want to do. What I would suggest is this: Batch convert your loops to full fidelity files (aiff or wav) and they will be available to import through the Audio Bin or File Browser whenever you need them. And then I would batch convert the one shot samples and create EXS 24 instruments with them in whatever groupings are logical for your workflow.