Unfortunately, Garageband does not offer the option to export a 320 kbps file. With that said, you can export WAV files from Garageband. WAV files can be uploaded to Digital DJ Pool just like 320 kbps MP3 files.
First save your project. Click on File and click Save. Next, click on Share and then Export Song to Disk. This will bring up a window where you can change the name, location, compression format and audio settings you want to set for your track. In this section, make sure you select WAV.
When that's all set, click the Export button. Your file will then save and be ready for you to upload :)