Uh, are you talking about elastic gussets -- triangular pieces with the point towards your feet? I wouldn't attempt it myself, I'd take it to a tailor who would probably shake her head.
I think it could work if you can find or make the gussets in a close-matching color. You'd have to use really strong stitching to keep them from ripping out, since elastic will pull harder on your material.
You might be able to cut down a maternity panel (used to switch regular pants to maternity style -- Hancock's online has them) to get the correctly-sized gussets. There will be a problem getting everything to look smooth (especially the points). Once you've got it right, make sure your seams are tacked down.
Real tailored pants usually have more than an inch or more of extra material on either side of the back seam which you can open up and use for more ease. Of course, any tailor will tell you that it is much easier to cut down a pair of larger pants to fit.