yes, it is possible - with one small exception.
First - you'll need an ISOLATOR (look up battery isolator) - this dvice goes between your alternator and your two batteries - it will keep them both charged, and also keep them from damaging eachother, which is possible if they're wired in parallel, and one of them happens to go bad.
Second, you'll need a switching mechanism - is your backup capacity to be used for accessories, or for starting?
Third, I've looked into it, and you could actually fit a standard battery, as well as a honda civic sized battery in the same spot we have a single battery, but the problem is that the battery tray would need to be removed and replaced with somethin custom.... thus the reason I haven't done so.
now, the caveat -
Unless you're willing to run some MAJOR wire form the second battery location to the front of the vehicle, starting form that battery will be challenging. While it SHOULD be OK if you have something the size of a jumper cable, my personal preference would be to run a cable capable of handling the full load of the starter and engine for a period of time. Switching will also be challenging, as if you want a remote switch (say, in the dashboard), you need to find a way to DISCONNECT the first battery when the second battery is being used. in theory, you could use a set of solenoids, but it would take some careful plannin and space under the hood for that system. The isolator won't handle this part - it only handles charging.
I've been wanting an auxillary battery for quite some time, but without anyone locally to weld up a battery tray to fit where the current tray is (or time to be without the vehicle while i mess aroudn with pulling the existing tray to use as a template), it's been on hold for over 3 years.....
I'm sure others will chime in, but if you're willing to buy the parts and find places to put them, i'm happy to help with the "how"....