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I've just tried this out, and have been unable to get it to work.
In theory GB reads .exs files (you'll find them in HD/Library/Application Support/Garageband/Instrument Library/Sampler/
In that folder there are two folders, like in Logic, one for the exs file (sampler instruments) and one for the samples (Sampler files).
However, the exs files that come with Garageband have been programmed to work with built-in Garageband instruments.
Garageband doesn't have the EXS24 and (AFAIK it can't be separated from Logic and placed in GB).
When trying to load a new exs instrument that's been fully copied into the correct locations above, GB won't know which instrument to open it with.
If you have an old copy of Logic Express 7, it came up EXS[/b]P[/b]24 which was a separate AU plug-in - so that would work. However, since version 8 Logic Express got the full EXS24 sampler.