I think the solution is the same.
Just run npm install node-red-contrib-s7 in the NR workspace of your online machine, and copy the whole workspace to your offline machine (important is the node_modules dir).
By workspace I mean the directory where your flows.json resides. I don't recommend installing nodes globally.
In principle, this is how I do it all the time, as our customers' production machines have no internet connection.
No npm cache needed.
"npm pack" is for building an archive when developing packages.
