Partial Resolving with bind?

[Michael]

Hi all,
I have the need to dynamically switch host resolution
in an existing environment.
I thought I can setup my own DNS for my zone, e.g. testzone.com
but I want only some systems listed in the zone file,
e.g. a.testzone.com
For b.testzone.com I want to refer to an existing
DNS.
I simply set up the existing DNS as forwarders,
but this is not the solution.
This one is used only for hosts in different zones
but for hosts in same zone I get a connection timeout.
How can I setup bind so that it asks the forwarder
for systems not listed in the local zonefile?

Thank you,
Michael