Assuming you can boot a live system I guess in your case it might be the best procedure, to perform your tasks in there and then transfer your downloaded packages over.
The following is regarding using a live-dvd (with a stick it should be more easy as long as you made it use the free space as persistent storage).
Boot your live-dvd and select 'try ubuntu', when finished attach a usb-stick. Open
gnome-disk-utility, find your usb-device and unmount it, also write down it's device-path (for this example we will use /dev/sdb1).
Now open a terminal with
Ctr-Alt-t and type
sudo mkdir -p /var/cache/apt/archives/ this will take care that this directory truly exists.
In the same terminal, you now mount the usb-stick to that point
sudo mount /dev/sdb1 /var/cache/apt/archives (be sure to use the correct device).
So now we will add the repository holding the software you want:
- for classic deb-archives:
sudo echo "deb http://archive.getdeb.net/ubuntu xenial-getdeb games" >> /etc/apt/sources.list.d/getdeb-games.list add the pgp-key
wget -q -O- http://archive.getdeb.net/getdeb-archive.key | sudo apt-key add -
- for ppa's:
sudo apt-add-repository someppa/ppa
Now we update our package lists
sudo apt-get update and download the packages
sudo apt-get install -d warsow; the -d options makes apt-get only download the package and all dependencies.
Voila finished, all your needed packages are now on your stik. Reboot into your normal system, attach the stick, open a terminal and cd into the stick and there issue
sudo dpkg -i *.deb to manually install all packages and then run
sudo apt-get install -f to clean everything up.
This procedure can bring in newer versions of already installed packages depending what repositories you use, so be careful, what you download and install. With third-party repositories there is also always the probability to catch yourself a flu or a virus, so think about what you do.