If you have a commercial license to FiddlerCore, I can make available to you a version of the exporters (BasicFormats.dll) which works properly with FiddlerCore, such that you can achieve your scenario with FiddlerCore alone.
Alternatively, you can do either of two things:
1> Capture the traffic with FiddlerCore. Save it to a SAZ file and automate Fiddler so that it loads that file and exports the content from the SAZ file into the WebTest format.
2> Automate Fiddler (using ExecAction.exe) so that it captures the traffic and directly exports the content into a file in the WebTest format. See http://docs.telerik.com/fiddler/KnowledgeBase/ExecAction and https://groups.google.com/forum/#!topic/httpfiddler/4VK8lt4e3HA
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