So far Fiddler says it can only import unencrypted sessions from PCAP files.
Various tools and libraries support the SSLKeylogfile environment variable and log the necessary keys.
You can either have them inside the pcapng file or in a seperate file.
It would be nice if Fiddler would accept an optional file with these keys and treated sessions with a suitable key as unencrypted.
This would make things a lot easier in the process.