RAF does not provide data anywhere from <Extensions> node in VAST XML response. To get data from there, you will have to implement custom logic to request ads and parse ad response, and then provide ads data in required format to RAF using importAds(ads) method (for CSAI approach).
This could be solved much easier if RAF held cache for last ad response, or if it could provide that response string through callback function set via setTrackingCallback . But it's not.