What use case are you targetting; are you wanting to supply your own customized packages for installation or are you looking at a way to not have your development machines talking to the open internet? Something else?
You can use a symlink to point a directory inside of the current Packages path at an external location, so that might be one way to go about it. I’ve never tried that from a network share perspective though, so there may be some potential performance wrinkles at startup or something. Also in my (anecdotal) experience, under windows Sublime doesn’t notice when files inside of a junction’d directory are modified, which is a bit of an inconvenience.
If you’re using Package Control it’s also possible to set up your own package channel for packages. The list of your packages is integrated with the official channel (which you can also remove from the configuration so only your custom packages are visible), allowing people to install packages from your site as they would normally. In that case you would have to serve the files from an intranet (and I believe it needs to be accessible via https for security reasons).