With following command you are able to manually add an entry

# ip -6 neigh add <IPv6 address> lladdr <link-layer address> dev <device>

Example:

# ip -6 neigh add fec0::1 lladdr 02:01:02:03:04:05 dev eth0

Like adding also an entry can be deleted:

# ip -6 neigh del <IPv6 address> lladdr <link-layer address> dev <device>

Example:

# ip -6 neigh del fec0::1 lladdr 02:01:02:03:04:05 dev eth0

The tool ”ip” is less documentated, but very strong. See online ”help” for more:

# ip -6 neigh help
Usage: ip neigh { add | del | change | replace } { ADDR [ lladdr LLADDR ] 
          [ nud { permanent | noarp | stale | reachable } ] 
          | proxy ADDR } [ dev DEV ] 
       ip neigh {show|flush} [ to PREFIX ] [ dev DEV ] [ nud STATE ]

Looks like some options are only for IPv4...if you can contribute information about flags and advanced usage, pls. send.