Gaining

Does she WANT to gain weight?

There's your problem right there.

People look in the mirror every single day. It's almost impossible not to notice a significant weight gain. If you want her to gain 15 pounds, you'll have to do it with her full knowledge.

If you could make her gain against her wishes that would be stealth feeding, which is completely messed up.

Thanks for pointing out the overriding ethos of FF, guys. Feeding always has to be consensual. Thread closed.