First, disable DNSSEC validation for the bit. TLD. Namecoin doesn't support DNSSEC:

server:
    ...
    domain-insecure: "bit."

Then add the addresses of the public Namecoin resolvers to a forward-zone block for the bit. TLD:

forward-zone:
    name: "bit."
    forward-addr: 2001:41d0:2:a5d9::101 # French bit. DNS resolver
    forward-addr: 178.32.31.41 # French bit. DNS resolver
    forward-addr: 78.47.86.43  # German bit. DNS resolver

Reload Unbound to load the configuration changes.