Sure, it's possible - no guarantees, though. First thing, get rid of the dead, black, mushy trunks and roots, if you haven't already. Next, change the soil. In a shady location, un-pot, gently wash the old soil off the roots, then re-pot. Your new soil should be gritty and fast-draining, composed of particles approximately 1/16 to 1/8 inch in size. A lot of folks, myself included, use a mix of fine bark, small gravel, volcanic pumice, and perlite. I use a bit of charcoal, too. After you've re-potted, water the potting medium, then pour off any excess water. Keep the plant out of the sun so that it can recover for at least a week. This is about all you can do. Hope for the best (and keep an eye out for other nice Pachira at the supermarket).

Josh