Accordion Sound

I don't think so. Do a search for "accordion" on http://www.sf2midi.com. I found one that I liked called "all accordion.sf2".

You may need to buy a sound module with an accordion library.

Anything with the General MIDI Sound Set will include at least one accordion.

Actually, I personally thing the GB sax sounds a lot like accordion. 🙂

you should be able to drop midi files right into the timeline.

I guess by "midi sounds" you mean other instruments that you can use in GB? There are tons of soundfonts out there which you can use as instruments in GB (I gave you a link above).

For instructions on how to install them see
http://thehangtime.com/gb/gbfaq2.html#installsoundfonts

After you've installed them, choose any software instrument, in the "Details..." window select "DLSMusicDevice" as generator and click on the pencil icon to display a window where you can select the soundfont.

Of course it's possible.

There are a huge number of companies that make software instruments you can use with GarageBand. Apple Jam Packs are not your only option.
These companies include, but are not limited to,

Native Instruments
http://www.nativeinstruments.de/index.php?id=samplingline_us
IK Multimedia SampeTank http://sampletank.com
Linplug http://linplug.com
Garritan http://garritan.com
Propellerhead Reason http://www.propellerheads.se/
M-Audio
http://m-audio.com/index.php?do=products.list&ID=virtualins
Steinberg
http://steinberg.de/Categorysb0744.html?Category_ID=11&LangueID=7
Sonic Reality http://sonicreality.com