Can I modify the gearing on a 2009 Jamis Aurora?
You can easily put a 26t chainwheel on the front crank. I've done that on a couple bikes, because 30/39/52 makes for a stupidly redundant set of gears with anything like a wide range cassette on the back. 

It will likely take some tweaking of the front derailler setup to get it working properly, switching to a 50 from a 52 can help with that. 

It isn't an out of the box recommendation since you are slightly exceeding the manufacturers specifications for both front and total ranges, but in practice it
works just fine. I would recommend installing some kind of chain keeper like the 
Deda Dog fang. This will help avoid dropped chains when shifting from the middle to small chainring. 

As far as changing the rear cassette goes, you should be able to use a Shimano 9speed HG 12-36 and it's generally just cheaper to buy the whole cassette than individual rings. If your current cassette isn't worn, you can mix and match rings to build a custom cassette. However, switching cassettes doesn't really solve the problem as well as a smaller chainring would IMHO. A good granny will give you a few choices in climbing. If you go to just the 36t in the back, you've only added one lower gear and you'll be "stuck" there. 

If you are doing a lot of loaded touring, I would recommend replacing the entire front crank altogether. Standard road triples 130/74 BCD don't make a lot of sense for touring. A 110/74 or even a 9 speed MTB crank 22/32/44 will give a more useable set of gears. Personally, I would take a long look at the Surly Mr Whirly crank for touring. It uses modern bearings and you can get a replaceable spider to 
allow for whatever front gears you like.

IMHO, the old MTB standard 58/94 is ideal for loaded touring. 20/30/42 allows you to use a relatively close range on the back and still get all the gears you need. If you do go this route, you might need to replace the front derailuer as well. 

Keep poking around on Sheldon Brown's pages, there is lot's of good info there.