Look in the NEC, not sue what article but you might need to protect the existing K&T with a GFI. Ran into this about 8-9 yrs ago and that was the solution then, not sure if NEC has changed since.
The artical in the NEC for K&T is about 2 pages long and just says splices will be in an approved manner and wire will be in accordance with 310. I and my friend found nothing about GFCI, but I will double check.
Thanks
Rowland
