Sound quality output

exporting at a higher sound quality than 256 kBit/s, specifically at 320. Is this possible?

not directly.

instead of "Sharing to iTunes" use the Export to Disk menuItem, and export it as a 16-bit AIFF file.

drop the resulting file into iTunes, open iTunes' prefs and set the Import Setting to AAC and Custom where you can select 320kb/s:

finally, select your file in iTunes and use the Convert menuItem under iTunes' File menu to create your better quality AAC file

what is the final step to export the resulting AAC file? It is in iTunes, but I have no further way of exporting it!

you could either drag it from the iTunes window into a Finder window (or your desktop), or control-click the file and choose Show In Finder:

I followed the "Show In Finder" procedure, but when I selected it and moved it to my desktop, the resulting file was given an .aif extension. Is this what should happen?

you selected the wrong file; when you use the (poorly named) Convert menuItem, iTunes creates a brand new file rather than converting the one you selected. iow, after you "convert" you have two copies of the file in iTunes, an AIFF, and an AAC

if you don't see both, perhaps try using iTunes' search, and enter the exact name; two files should appear