How can I install CodeIgniter core (system) files using Composer?
This might not be what you are after, but the obvious answer is to create a composer.json file and require "ellislab/codeigniter" - this will install whatever is offered, as a complete package.
I have no experience with CodeIgniter, but I think this will install more than you need. I have not seen and way to just install the system directory. This would only be possible if CodeIgniter would offer this as a separate package - and they don't.
I've just stumbled over the project that does just that - codeigniter-composer. Never used it though, simply was searching for a similar thing and this popped out.