Create a new Kyma Sound file and put the Sounds you want in there. Optionally you can categorize them by collecting (Action -> Collect). This way they'll show up in their own category on the left of the prototypes browser (using the collection name), otherwise they'll be in the General category. Then choose File -> Open and select Custom Prototypes from the file type drop down list and open the Sound file you just created. Kyma remembers the custom prototypes, so you only need to do this once. If you want to add more Sounds add them to the Sound file and then import the custom prototypes again.