Overly salted sauerkraut
I'd actually add more cabbage rather than an entirely different vegetable. 

Despite what grandma believed, potatoes are not this magic salt-absorbing sponge: the advice to add potatoes to oversalted foods stems from the fact that more food with the same amount of salt equals less-salty food. In other words, potatoes don't absorb salt, they dilute it; and pretty much anything you add to a dish will serve that same dilution purpose, as long as what you're adding doesn't contain salt already. Thus, to dilute the salt in a vat of cabbage, I'd add more cabbage. Yes, this will result in some of the cabbage being more-fermented/softer than the rest, but depending on how long the sauerkraut has been going already, it may not be a noticeable difference in the finished product.

You can also try removing the salt by pouring off some of the brine that has developed and replacing it with water, but you'll be diluting the cabbage taste as well as the salt.

Another option is to just let the sauerkraut ferment, knowing that it'll take longer because of the increased salt levels, and when you're using it, rinse the bejeezus out of it.