The age difference is too great to mix the two litters but you can let the you let the younger kits nurse on her, she may need to be held to prevent her from harming them and I would not leave them with her unsupervised.
The kits will be close to the same size so the older litter won't hog the milk and it should be alright. If you only want to add a few kits to the older litter, pick the larger ones that match their size.