Do you still have the old GarageBand version installed or can reinstall it from the installation media? Then create the sampler Instrument in GarageBand '11. if you save sampler instruments in garageBAnd '11, you can use them in GarageBand 10.0.
With GarageBand '11 installed you can even use the SoundEffect instruments directly in GarageBand 10, if you once saved a Nature instruments patch or RadioSounds patch in GarageBand 11. It will appear in the "User Patches" of the library. You can even drag new samples onto the keys. You just cannot save the sampler instrument before you quit GarageBAnd. So you have to finish the project in one go.
For example, I used the Radio Sound instrument in GarageBand 10 and added new samples, using a saved "User Patch" created in GarageBand '11, version 6.0.5.