First, be aware that you get two different sets of GUI configurations with most of the instruments in GPO5.  The 'notation' variant is intended for notation software like Finale and Sibelius, and for all but the new GOS 'Small Group' strings pushes all the key switches to the very bottom/left of the spectrum (starting with MIDI Note 0).  Meanwhile, the regular, or non-notation variant is better suited for live playing or tracking DAWs, and it already pushes the keyswitches up to just below the bottom range of a given instrument.

Note, if you only see Notation versions of banks in ARIA (Instrument names in ARIA are preceded with an "n"), then you'll need to reinstall  the library, and this time choose options to install BOTH the regular and notation banks.  Don't worry about it taking more disk space, as the banks are really small xml files, and the 'large stuff' like samples are shared across the board.

Under the hood, both the Notation and Regular bank calls to GPO5 instruments use the same sfz file.  The main difference between an n-instrument and a regular one is simply the GUI that pops up under the ARIA Controls tab, and the key-switch placement. 

So, if you want the key-switches just under the playable range of an instrument, then you can already get this by using the regular variant of instruments.

If for some reason you want to move the key-switches to somewhere less common (I.E. to the RIGHT end of the keyboard), or maybe you'd rather use CC messages instead, there are unofficial (as in not supported directly by Garritan) ways to tweak and customize many aspects of the Garritan Libraries.

Before going to all the trouble you might consider just getting a larger controller (For me 5 Octaves or 61 Keys with a nice bank of programmable MPC pads is a good compromise between something that's not too bulky and heavy to carry around, yet still fits on my computer desk...but if you plan to use it regularly in demanding live settings it might be worth it to go for 76 or 88 keys), OR, if you want 3 octaves or less for the sake of a smaller, more portable board, then I definitely recommend getting one with plenty of programmable pads/buttons (I.E. a bank of MPC pads) that can be set to send key switches (and other things).  You can also take advantage of 'presets' in many MIDI controllers to make dealing with keyswitches a little faster/easier.  If you're a proficient pianist/organist, you'll probably be happiest with 5 or more octaves (find/make the space for it).

MPC pads (or other banks of programmable buttons) are nice to have, even if you don't plan to bang on them to get 'percussion' sounds.  They can usually be set to send all sorts of things such as key switch notes, program changes, or CC events.  I often use mine with an 'organ stop' philosophy behind the way I've got them set up.  I also use a bank of MPC pads to remote control things like step input in Cubase, Finale, and Sibelius.

It is possible to edit the sfz files from your Garritan Library in a standard text editor, as they are simply text files that map out and define how samples or synth oscillators get triggered and/or altered over time.   With this approach you could edit existing key-switch groups, or create new ones.  ALWAYS back up a sfz before you examine/edit it.

While you can tinker with the sfz files, be aware that (as far as I know) we shouldn't mess with the bank.xml or the GUI files, as altering these can break the registration/encryption protocol for the Garritan Libraries.  With this in mind, be aware that some custom tweaks to an sfz file might break or alter functionality in the 'controls tab' of the aria player, so if you intend to experiment always backup things and take plenty of notes about your changes.

You can find sfz opcodes here:  http://ariaengine.com/overview/sfz-format/ and there is a really good support community for the ARIA engine here:  http://www.plogue.com/phpBB3/