Ways to fatten her up?

Poor Keller had a litter of kittens, got ringworm, and now just two weeks after finishing her RW treatment has an URI. She is really thin looking and I wondered how I can help her fatten up a bit. Maybe I should just give her some time? She eats Wellness Core mixed with Wellness kitten and whatever wet I have on hand. She has been getting some tuna juice that I use to disguise her meds in (she is impossible to medicate). Is it safe to give her some tuna? She just looks so run down to me and I know time will help that.