This can be achieved with the network manager CLI. Eg:
nmcli connection modify "$PROFILE_NAME" +ipv6.addressed fc00:b:c:0::5/64 +ipv6.routes 'fc00:b:c:1::/64 fc00:b:c:0::1'
Once the profile has been modified reactivate the profile with:
nmcli connection up "$PROFILE_NAME"
Check the result with
ip -6 addr and
ip -6 route.