what files are you trying to add? LTIB creates a filesystem and all pkgs are installed on ltib/rootfs folder. In the other hand, oneiric is an ubuntu filesystem, with different filesystem tree structure; if you need to install packages in Ubuntu, you need to use 'apt-get install' on the embedded device. In case you need to compile code, you need the developing tools (gcc, as, etc) and compile directly (no cross-compilation).
Leo