How can I add previously downloaded .deb files to apt using command line so that they will not be downloaded again when I use
sudo apt-get install?
Copy it to /var/cache/apt/archives with the following command:
sudo cp filename.deb /var/cache/apt/archives/
You may have to rename the deb files according to the cache, so start to download it with
apt-get, get the name of the cached file from
/var/cache/apt/archives/ then rename your file and copy it there overwriting the incomplete download and start
Of course you don't need this if you are copying from an old cache :)