When Kdat formats a tape it writes a single file at the beginning of the tape. This file should only occupy a single tape block. The contents of the file are:
(9 bytes) The string literal "KDatMAGIC"
(4 bytes) The file format version number (currently 1).
(4 bytes) The length in bytes of the tape ID string.
(n bytes) The tape ID string. The format of this string is "<hostname>:<seconds>", where <hostname> is the full name of the machine that the tape was formatted on and <seconds> is the number of seconds since the epoch when the tape was formatted.
The tape ID is used to locate a file, with the same name, in the $HOME/.kdat directory.
Each of the remaining files on the tape are plain-old tar archives. You should be able to manipulate them directly with GNU tar. Even non-GNU tar should work for non-incremental backups.