Hi, I've never owned a ferret before but I've done a TON of research, and my friend (and neighbour) has three of them, that are always around. I just got my first ferret tonight, and she has some issues... I knew this when I bought her and the weeks ahead that I've been talking to her previous owner, but I need some true expert advice here. 
She is 3 years old... last year a box was dropped on top of her, and she has limited use of her back end. Her actual legs work totally fine, but it's like her mid section of her spine is paralyzed. She can climb and walk on fabric, but hardwood and concrete give her troubles, and even when walking on fabric she falls over a lot, just onto her side. She doesnt seem to be in pain at all, and shes learned to deal with this completely (just flips onto her back and uses her front momentum to push herself back upright when she falls) but I want to make her life as easy as possible, while still being able to do things with her and give her an exciting life outside of her cage. Does anyone have any advice for this, or has anyone seen this type of injury before??
Also, my second big concern with her is that she's TINY. Her previous owner took her to the vet last year and she weighed in at 0.9 pounds. I can feel each and every one of her bones and I'm scared that she's too brittle and will hurt herself. Is there any way I can get her to gain weight? Or muscle? As of right now she eats ferret food, and has a couple supplements (ferret vitamin stuff) that I've read are not good, so I dont know if I want to give her that...I've been told she's very picky, so please help! I'm reading everything I can and still doing lots of research to try to figure this out too, but any help would be very much appreciated!! 
Thanks in advance!!

Hello and Welcome to the forum! Thank you for taking her in. 
I feed mine soupy 3x a day. I soften kibble then purée it in a food processor. I add a jar of chicken baby food(1st stage), heavy whipping cream, an egg and enough water to make it the consistency you want. I freeze any extra and keep enough in the frig for several days. I use a syringe at first if they won't eat it on their own. Usually it's not long and they'll eat it on their own. 
I don't remember if you said you've been to the vet but I would and have them do a blood glucose test. The test is inexpensive. 
No on the ferretvite. Full of sugar and sugar can cause insulinoma. 

 
Fuzzbutts Rule!

Hi there.  What kind of ferret food is she eating?  Cheap stuff is not adequate.  Vegetable and grain fillers can't even be digested.  A good kibble supplemented with soups will help a lot.  We make soups with ground premium kibble and raw poultry.  Once they get the taste for soups you can add different things as needed and to give variety.  The formula Bandit suggested should help put on some weight.

THE DOOKS OF HAZARD

She is eating Marshals Premium Ferret food, and a tiny bit of 8-1 Ultimate chewy things... she doesn't like that very much though and kind of eats around it. I haven't taken her to a vet yet as I only got her last night, but I will definitely ask her previous owner when her last visit was. 
Thanks for the help on food, I will definetely try that!