The only way to enter project-level information is by using the Project Description. You can edit the project description (or any project details) if you are have the Project Administrator permission for a project.
That said, it sounds to me like you might already be using some add-on to acheive this. I can't tell if you're already capturing project-level information using project-level fields - are you using an add-on like Profields? If you're already using Profields, I believe you can create different schemes to use for each project to capture different fields for each project, but I've only trialed the add-on and it's been awhile since I did that.
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