many COULD do it, but you're gonna have a hell of a time finding someone who would.
look how much ford was out with the pinto's fuel tank problems. no sane mechanic is going to set himself up for that kind of a liability for the couple hundred bucks he'd make installing it.
plus on the newer (and smaller) SUV's you are really lacking for anywhere to put it other than in the vehicle, on top, or on the bumper. so if they guy installs one inside and you spill fuel while filling and burn your family alive, he loses everything he owns.
I've added extra tanks on older trucks with no problem. on my 71 ford it came with a 25 gal under, and a 17 behind the seat.
I made room for another 25 beside the existing by converting my exhaust to stacks, then put the new tank in the frame opposite the existing and connected them with an upper and lower hose. but since it was my own truck and I knew my own work, there was no risk.
your best, and easiest bet is to have a rack made that holds 4 or 5 jerry-cans(I made one for a friend a few years back. unfortunately the drawings were lost in a HD format or I'd send them too you) . the best way to go with this is if you have a tow package, remove the hitch and have the rack mount by sliding into the receiver. that way you can have your gas when off-road, but not have the rack in your way in town.
you can also easily adapt a electric fuel pump with a hose on either end, and a cig-lighter adapter so you can plug it into the dash for power, to speed up the refilling process.
you also should build a locking system for the cans (a bar that slides through the handles works well), and a small locking toolbox into the rack, to store the pump and hose in. that way you won't have to smell the gas after using the pump.