With the release of Soundpaint 3.0 in 2024, the USI Editor now allows you to layer up to 127 samples on each key using velocity layering. Check out our article here on how to set up velocity layers for your own samples.
The Soundpaint USI Editor does not currently support importing your own Round Robin samples.
More information on the new USI Editor can be found in our 3.0 Walkthrough: