There is an android app, Passwallet, that is able to interpret pkpass files intented for the apple app Passbook (https://play.google.com/store/apps/details?id=com.attidomobi
You can download the full specification for the .pkpass bundle from here. The pass content is stored in a JSON file named pass.json. The .pkpass bundle is a zip file containing pass.json, the pass images, optional locale files and a manifest file.
The manifest needs to be signed with an Apple issued Pass ID certificate. However for Android, or any other third party App, everything required to build the pass can be read from pass.json and the bundled images.