Windows Phone 8.1
The Wallet in an early Developer Preview included support for .pkpass but the later versions does not have the support (the Wallet still support the passes but the support for the .pkpass extension seems to be missing).
See below for other ways to open .pkpass.
Windows Phone 8.0
There are third-party apps available like Wallet Pass, All ...
If you just want to see the data, and have an android device, transfer to your Android phone and download a app called SQLite editor, then find your .db files and try to open, I could open some db files on my phone