You can import your own audio samples (wav-files) into Omnisphere by clicking on “sample” on a layer, then clicking on the text “empty” as seen in this image.
After you do that you get into the sample browser mode, and to the top right there is an area where you can drag and drop an audio file from your computer.
After you have imported any samples (and yes, you can even import a complete folder with audio files at the same time), you will be able to access them as “samples” in any layer of the presets you sound design in Omnisphere.
Note: there is a 2nd option to import audio samples. Click the “Utility” button in the interface, then choose “User Audio“.