GarageBand is a limited version of Logic, with Logic’s EXS24 sampler under the hood. Here’s how you can import EXS instruments in GarageBand with earlier versions of GarageBand. I haven’t found a way to do it in version 10, I’d be grateful if someone can point me in the right direction!

So, in earlier versions:
Go to: [user]/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Instruments and drop the EXS file in there.
On Snow Leopard and higher you need to go to “Go” in the Finder and hold Alt (Option) to make your User Library visible.
Go to: [user]/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Files and drop the sample folder in there. Give it the same name as your instrument!

Go to: Library/Application Support/GarageBand/Instrument Library/Plugin-In Settings and then open the folder that fits the instrument you want to import.

Copy the file that fits the instrument to a folder with the identical name in:
[username]/Library/Application Support/GarageBand/Instruments Library/Plug-In Settings/
Create a folder with that name if it isn’t there.

Give the copy the name of the EXS file you want to import and open the copy with TextEdit using control-click or right click. You will find the original instrument name in the code. Replace it with the name of the EXS file you want to import.

I picked “Horn Section.pst” and after replacing the text it looks like this:

Open GarageBand. If it’s already open, close it first and open again.

This is what I miss in version 10:

Go to i – track info. Select the instrument class that suits your instrument, then select the subclass. In my case: Horns -> Horn Section.

Open Details. On the right of Generator, pick your instrument name. It happened once that the instrument wasn’t there or wouldn’t load, I then copied another pst file, renamed it and replaced the instrument name and that worked.

In GarageBand 2 it’s a pop-up window:

In GarageBand 4 it appears in a panel on the right:

In later versions it’s on the left, like in GarageBand 10.

If the instrument loads, you can tweak the instrument channel. Next, save the Instrument.

GarageBand will write a cst file for the new instrument in [username]/Library/Application Support/GarageBand/Instrument Library/Track Settings/Software/

If you want to move the instrument to another computer, get the sample folder, exs file, pst file and cst file, copy them to the exact same locations and GarageBand will find them.

In GarageBand 10 you can find the instruments you imported using older versions of GarageBand under “User Channel Strip Settings”.

Enjoy your new instruments!