Short answer: You can't.
Long answer 1: You can, but it involves some technical explanation I won't get into here (I'll give you the overview)... You create a database specifically for that content, then export it all to an external drive (of whatever kind - hard drive, stick, SD card). When you want to use that content, connect the device and rekordbox will show it. You could store the database for that content on that export drive so you can easily swap to that database as the primary at any time.
Long answer 2: Why? There are so many features within rekordbox allowing you to filter down what you need, why would you go through the unnecessary effort to separate the content and make it less convenient?