Mirroring is what i have in mind you know like the linux one, you can actually mirror the packagist and i suppose hosted them on your company network Packagist mirror from here, and the suggested is Packagist Mirror github after that you specified the custom repositories in your composer.json that lookup on your local network Repositories
there is some other way that i found in this stack over flow discussion, you can download the specified package that you want and load them up as the discussion suggest, see Artifact in composer
edit you can also see this one handling private packages this might be the one you want