Constant weight in rear of car over rear wheels

Having additional weight won't damage anything. It's like having a couple of kids in the rear seat. It will wear the rear shocks a little. It will also decrease the MPG's. Additional weight added to the rear won't do much for traction unless your Fit is four wheel drive. Get a set of four snow tire if you don't have them yet. They will increase traction a lot more than weight will.

any car in snow and slush over 2 inches deep will have problem with traction...as mentioned previously snow tires will help..but also the the tread depth of your current tires