Add Bootstrap into Zen sub-theme
Does anyone know if I can only add these two lines of code to setting instead of downloading every single bootstrap file?


No, you can't do that. That's a setting from, I would guess, the Bootstrap theme itself. It allows you to choose which version of the framework to use for that theme. The Zen theme would have no reason to add Bootstrap, so no reason to use those settings.

Continuing to use Zen as a theme/sub-theme, and replacing its framework with Bootstrap, doesn't sound like a great idea. You'll have problems/headaches upgrading, and plenty of Zen-isms to remove/re-implement to get it to play nicely with the new framework.

Given that all of that work, and more, has already been done for you, setting up a new theme based on Bootstrap is worth re-considering IMO.