JetS3t doesn't have a way to read the AWS credentials file, but I believe these files use the standard INI file format  so you might be able to:
- find an INI file reader library
- use that library to parse the AWS credentials file and pull out the per-section settings you need
- feed the credential information intoJetS3t's ProviderCredentials object.
I don't have any experience with parsing INI files in Java, but ini4j  comes up high in Google searches.
Or, if you are already using the AWS Java SDK library as you probably are, it seems to me you should be able to create its DefaultAWSCredentialsProviderChain class  to have it find and parse the AWS settings for you, then fetch the credential settings you need from that class to provide to JetS3t's equivalent.
Hope this helps,