hey there,
for me it took a few months but after about 3 months post transplant I started putting on much needed weight.
Just like mentioned above you aren't using as much energy to breathe, fight infection and so on so you can store some of the food and put on some weight. Also you are able to exercise with more ease and that helps to put on muscle and thus weight.
As for pancreatic insufficiency. I was and am pancreatic insufficient and diabetic. I still take meds but not as many as I had to before and I still gain weight. I have gained about 20 lbs since my transplant and am at a healthy weight of 127lbs at 5'7" height and that is about 10 months post transplant.
I also will say that most of the people I know post Tx with CF it took them a few months to start putting on the weight but it did come eventually.
Take Care,
Lindsey