I have two Ubuntu 18.04 machines one have access to the internet and has ROS Melodic installed with some packages and dependencies, the other, however, doesn't have access to the internet, now my question is how to pack my ROS Melodic (on the online machine) using snapcraft, synaptic or whatever and make it available offline for the second Ubuntu machine?
Done by downloading the Debian (.deb) file of Synaptic Package Manager and its dependencies in the online machine, send them to the offline one install them and set it up to accept packages from http://packages.ros.org/ros/ubuntu, open Synaptic Package Manager and mark (by hand) every package you want to install (by clicking on origin and select bionic/main (packages.ros.org), right click on the package and select "mark for installation"), after done marking, from file click on generate package download script you'll get a .sh file.
now take that .sh file to the online machine, create a new folder place that .sh file in the folder (so you'll not end up in mess like I did) and run the .sh, when it's done you'll have all the .deb files of the packages you marked, send those .deb files to the offline machine, open a terminal in the folder that contains the .deb files and install them by typing sudo apt install ./*
and you're done!