That's a very interesting question. FiddlerCore (and pretty much everything else on your system) will use the default OS DNS resolver to resolve hostnames. Fiddler/FiddlerCore allows you to override the address used to connect to a target server (via the X-OverrideHost Session flag) but I'm not aware of anything in the .NET Framework that would allow you to make DNS queries of an arbitrary DNS server-- you'd have to include a DNS Client in your extension that allows you to use the target DNS server for resolutions. (See e.g. http://www.codeproject.com/Articles/23673/DNS-NET-Resolver-C)
Can you elaborate on why you would want to do this?
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