Foods to promote weight gain

My female kitty has been a scrawny little thing all her life - not unhealthy thin, but long and slender thin. She kind of looks like a greyhound in that her upper body is big and her lower body is small. I'd say she's about 6-7 pounds. No health issues ever, either - going on 13 now, she's just as bright-eyed, bushy-tailed, energetic and limber as she was when she was 5. No worms, no sickness...just a little girl with a big appetite, just like her mum.

As said, I don't think she's a bad thin, but I also think it wouldn't hurt to maybe help her gain a pound or two. Is there anything I could feed her or add to her food that might help put another pound on her? I'm not by any means trying to fatten her up, but I think she's rather on the low end of normal. Perhaps I am misguided in the belief that I need to help her gain weight in the face of what is obvious fast metabolism on her part.

I've heard that adding a little olive oil to kibbles can help with weight gain and help loosen up hairballs...and make the coat shiny. Would this help?

Had her blood tested last year around this time and everything was pretty good. I know, she should get tested a bit more frequently, but money's rather tight right now and has been pretty much all this year. She'll be getting another one very soon, though.

Anyway, her weight is not a new concern - she's been a petite little thing her entire life. She's loved food all her life (has never been deprived of it either). And I'm always checking her for weight loss; she's so tiny that if she loses so much as a half a pound, I feel a noticeable difference and feed her a bit more until she gets back to her weight plateau of 6-7 pounds. I also do the thing where I lightly pinch the skin on her neck to see if it falls back into place as it should, I check her for anything abnormal, you get the idea. I watch all three of my trouble makers like a hawk. So if my girl does somehow have something that has been overlooked her entire life, I don't think it's going to hurt her if she's had it for 13 years and she acts as spry as when she was a wee thing.

I find it absolutely incredible that my girl is a skinny kitty, but her brother eats like a normal cat (and sometimes surrenders shared treats because his sister commandeers them) and he's bloody 20 pounds! He looks like a miniature cow; feels like one too when I try to lift him. And he's healthy, as per the vet's examination.

But thank you very much for the suggestion of kitten food; I'll run that by the vet and see if that sounds okay to do and if there's a brand they suggest. It's not unhealthy or bad to give a And while I know (or I think I do) that cats are lactose-intolerant, I have read some sources saying that another weight-gain food to give cats is milk or cream. Is this true? I know my cats would love milk and would probably do cartwheels for cream, but I don't want them being ill from it.