Feral kittens, often born into a colony, can warm up quickly to house cats. In fact, the kittens will probably bond more easily with your pets than with with you. Having adult role models can help the feral babies adapt to indoor life and the safe, loving home you've provided.
Health Check
First, make sure the kittens have a clean bill of health from your vet. Before they meet your other cats, they'll probably need a rabies shot, deworming treatment, and medicine to kill fleas and ticks. The vet also might recommend testing for feline immunodeficiency virus and feline leukemia, which can be contagious to other kitties.
According to Alley Cat Allies, a rescue organization, feral cats don't have a higher risk for these diseases, but it's always a good idea to make sure you aren't risking your other cats' health by bringing new kittens into your home.
Safe Space
Give the feral kittens their own safe space with food, water, a litter box and a pillow or blanket. Confine them in a small area, such as a bathroom or closet; if you put them in a larger room with ample hiding places, they could vanish for days. Make sure the kittens can smell and communicate with your other cats -- and vice versa -- through a door.
Read the Signs
Monitor how the feral kittens and your house cats interact through the door. Signs they aren't yet ready for more formal introductions include hissing, growling and reaching under the door to swat at each other. If they're chatting or even cooing at each other, though, you can try bringing them together. Make sure you're in a position to intervene if any of the cats becomes overly aggressive or frightened.
Intermediate Steps
If you notice continuing hostility on either side of the door, try a different approach. You could move the kittens to another room -- still keeping them in a confined space, such as a large dog crate, where their options for escape are limited -- and let your other cats explore the area that the kittens occupied. This can help them become more familiar with the kittens' smell and reassure themselves that these new roommates aren't a threat.
If you do have a dog crate, move it to a room that all the cats can enter, and put the kittens inside. This can allow the cats to check each other out visually. Make sure the kittens have enough room in the crate to retreat, and give them a place to hide if they feel overwhelmed.
Family Ties
Adult cats often bond more easily with kittens than with other adults. Feral youngsters, who usually begin their lives in a feline colony, can adapt quickly to life in a multicat home. Don't be surprised if your adult pets "adopt" the kittens, grooming and nurturing them.