I'm really not sure whether you're asking about
:
1) if you should buy a 5 string set to get appropriately gauged strings
2) you're trying to downtune an E string to B and your electronic tuner won't register
3) you physically can't fit a B string into a tuner on your bass, or
4) you have a B string that won't fit into the nut.
If you want help, you gotta help us out a bit and explain the problem a bit more clearly.