IPK archives which are used from "opkg" package manager are essentially archives which you can easily extract:
[bash]
$ mkdir package_contents
$ cp <package>.ipk package_contents
$ cd package_contents
$ ar x <package>.ipk
[/bash]
Will extract the contents into the folder package_contents. You'll find the contents now in data.tar.gz by calling
[plain]
$ ls
control.tar.gz data.tar.gz debian-binary <package>.ipk
$ tar tf data.tar.gz
[...]
[/plain]
