Need Help introducing lonely kitten to family of 6!

I am currently fostering a mom and her 5 babies. The babies are 5 weeks old. Someone from my rescue begged me to take a 6 week old kitten who is alone. They thought socialization with a mom and her siblings would be important for her growing up well socialized. I put them together. (I hate putting an unknown kitten with my healthy 5- but my rescue manager gave me know choice!) The kittens are fine with the new addition. However, Mom is not at all happy. For the last 2 hours she has been hissing, growling, even going after the kitten a few times. Poor girl is terrified! I know this is not the ideal way to introduce new kitties to each other, but in this situation, is their an optimal way in including her in the family? Will mom adjust? Will the kitten be safe when I am not in the bathroom to watch mom? Help!

I would not leave that kitten alone with the mom. I think I would even give up the idea of introducing that kitten for the time being.

Okay, this may sound strange, but just consider it...

I was in the same situation about a year ago. I had a cat who had just given birth to 3 kittens about a month before when I found this poor kitten near the expressway. When I brought the new kitten in, I thought "Perfect, he can just feed off the momma cat and no problem, right?" Wrong. A big wrong. The momma cat was behaving exactly the way yours was. She was hissing and even trying to hit the poor new cat. I tried to sneak the kitten in a few times but every time, the momma cat would see even just a hint of orange (the color of the new cat) and she'd go nuts. Yeah, I know cats are color blind, but I swear, this one new orange when she saw it! Anyhoo...I was about to give up when I had an idea. I knew that I usually could get away with sneaking the new kitten in with the three others for about 10-20 seconds, while keeping the momma distracted with a toy or a jingling noise (nothing loud enough to startle her though, because then she would get up). So, I sneaked the kitty #4 in, while distracting the mother, and then placed a TOWEL over the kittens, successfully hiding them all from the momma's sight!

Now, I thought this surely wouldn't work, but it did. The kittens didn't seem to mind having a towel drapped over them (I sat there for the most time, holding the thing up a bit to let them breath) and the momma didn't notice the new kitten. I did this and then in between (in small intervals of time), I would allow the momma cat to smell the kitten (if the kitten was in my hand, she would hiss but not attack. I think a cat knows your hand and won't attack; but if a kitten is alone, well that is a different story). Eventually the momma cat got used to the kitten, but it took a while, about 2 weeks. By the end, all four cats were happily eating from the mom, together and towel free!

There's one thing you can try, but there's a possibility she just won't accept the kitten at all. Try taking mom out of the room for awhile. Before you go back to the kittens, take a towel or something and rub it all over mom really well. Take it in and rub it on each of the kittens, and then finally the new kitten. That way their scent will mingle and the kitten will smell like one of hers. It still may not work, but it's certainly worth a try.
I've had three foster mommas this year and have taken in 6 abandoned babies (but lost 2) while at least one of the moms was nursing... the mommas I had accepted the new babies without any problems at all, but obviously every cat is different.

Thank you all for your input. I few hours after we placed them together, we gave up the attempt. Mom was still swatting, biting, growling, and hissing. This kitten had to hide from the mother. Mom was even getting stressed out with her own kittens. If she saw this kitten from across the room, she would hiss or growl under her breath, without even smelling her. Since I couldn't supervise, we were too worried with everyone's physical and mental health. It may have eventually worked out, but we didn't want to take chances without proper supervision. I have successfully seen many orphans join existing families of nursing moms/babies. However, I had also never seen a mother this upset with a 6 week old kitten! While this mother is very sweet with me, she isn't even a very motherly type cat to her own kittens!