How can I help my underweight rabbit gain weight?

I have a 4 year old rabbit (dwarf lop). She is 3.2 pounds. I can feel all of her bones and she's so frail and fragile.
Should I be free feeding her pellets? I have been giving her pumpkin through a syringe 2x a day (about 2 tbsp each syringe), pellets, lettuce, and hay. She has a good appetite. I have only have her for 4 days.
What else can I do for her?

[ireneo]

Do you know why she's underweight?  If she's extremely underweight, overfeeding her can kill her. An emaciated animal won't even have the strength to process real food. The first step should be a visit with the vet to find out if she's sick and then get recommendations on proper diet to help her get back to normal.