When I use Yeoman to scaffold new web apps and it automatically runs bower install && npm install for me, I notice it takes a few minutes to download all packages / dependencies.
Is it possible to simply copy over all packages from a local directory instead of having them all download?
Unfortunately this isn't possible. Yeoman isn't in control over where and how the dependencies are installed. Yeoman only orchestrates the setup (scaffolds) of the development environment, and the dependencies are defined in package.json file, which is not part of yeoman, but comes as a part of the template used.
All yeoman then does is invoke the npm install command that will fetch all of the dependencies.
Besides it is considered a bad idea to use global packages when dealing with dependencies defined in package.json. For instance if you have two modules that depend on two different versions of the same global module, you have (and could have serious) problems.
This is why when you use npm, all modules are generally installed locally.