I checked the Jira Cloud REST API, and didn't find any operations that read or update the "favorite" (star) setting for a project -- per user, or otherwise.
Since it's not supported in the API, it's unlikely that a Marketplace App will support such a feature either; sorry.
I believe the only way for an Admin to do this is manually -- impersonate each user and favorite the project(s). Note that users will be notified that you logged into Jira as them (so be sure to proactively message them about what you're doing). This will be a LOT of clicks. Sorry there's not a better answer.
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