Adding Existing Users to all Sites in Multisite

I'm looking for a way of adding a selected existing user, to all of my sites in a multisite network. So, for example, I have an existing user of jsmith and I need to add that user into all sites, without having to go into each site manually and add them and ideally, also select their role at the same time. Is this possible? I can't find a plugin to do what I need.