Yes!!! you can use EXS Instruments in GB. For each instrument you will have an instrument file with the suffix .exs and a fiolder with the samples for the instrument. Place the .exs file and the samples in the Sampler Instrument and the Sampler files folders in the Library/Application support/GarageBand/Instument Library/Sampler/ Directory. Your Exs will now show up as software instruments in GB.
Simple as that.
I will happily trade you walking you through the procees fo a decent trumpet exs instrument.
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Jay Shaffer
Mac Audio Guy
Author of "The MacAddict Guide to Making Music with GarageBand" mag@jayshaffer.com http://macaudioguy.com/