There is actually no existing easy way to install packages whithout a network connection on the Linux computer.
As all the packages are available from packages.ubuntu.com and as each paquet contains the list of his dependances, it should be possible to do a software, which will work both on Linux and Windows.
This software would get all the dependancies needed by the package the user wants to install, and then he would be able to install these packages automatically (in the right order).
This is not a very usefull idea, but it's not useless ! I really know peaple who still doesn't have a network connection, or to install Wifi drivers it's not very conveniant to move you PC in a public internet location...