Unbound: Forward certain queries to DNS servers assigned by a PPP server on WAN

[thiasaef]

Is there a way to forward certain queries to a DNS server assigned by the PPP server on WAN?

Basically, this:

forward-zone:
  name: "flickr.com"
  forward-addr: 217.237.148.22
forward-zone:
  name: "youtube.com"
  forward-addr: 217.237.148.22

but instead of using hardcoded IPs, I just want to use one of the DNS servers assigned by the PPP server on WAN. Is this possible, or is there any other way to achieve the same thing?