Sean submitted a new resource: Spigot Plugin Downloader - Easily download spigot plugins on your server through a simple CLI app Read more about this resource...
I believe that is not possible. It simply gets a direct link to download as seen in Code: You Must Register to View Code and for premium plugins, Spigot returns a 403, requiring you to login to even view them.
As Jayson said, unfortunately not. I may add an option to input a cookie string so you can download premium plugins.