Depends on the hood, but most can be cut down, whoever be doing the cutting they just have to make sure the inside hidden edge is being cut. If someone is handy all they need is a grinder, Norton 4 1/2" steel cutting disk (sold in Home Depot) and Norton 20X flap wheel about 80 grit to knock off the burrs after the cut is made.

Make sure to wear eye, ear and skin protection when cutting or grinding. If you hire someone, they will charge you around $20-25 to make the cut and clean it up.

Good luck.

P.S If you look on youtube, I'm sure they would have a video or two how to make the cut.

http://www.therangehoodstore.com/36-inch-wall-range-hood-zlkn.html

Super people at this outfit, call them and ask for assistance. Good prices also.

Thank you so much, chiflipper, for the tip. I think I finally found the perfect solution to a similar problem, and at the perfect price. I had almost settled for a standard under cabinet model with a valance above because there's not room for a 12" cabinet. Whew.