If you just need to read the contents of the tar files, have you looked at the Apache Commons VFS Project? This has the capabilities treating tar files as a virtual file system:
http://commons.apache.org/proper/commons-vfs/filesystems.html#Zip_Jar_and_Tar
I must first preface this by saying I have never had to read tar files, but I have used the VFS libraries for other protocols.
Just in case you aren't aware or for others who come across this post, the commons vfs library jar files will need to be added to your Ready API installation, see the "External Java libraries" section of the page:
https://support.smartbear.com/readyapi/docs/testing/scripts/about.html
