How can I let Pro Sites users upload their own themes or plugins

Hey guys, sorry to bug you on Christmas Eve. I'm just wondering if there's a way to allow a Pro Sites subscriber (or user, whatever we want to call them) the ability to upload third party themes and plugins.

I don't see any mention of it in the documentation or videos and Google hasn't helped me come up with a way either. I understand the security risks that this could create but that's a separate issue I'm prepared to handle.

In case it matters, I am a developer with a lot of PHP and backend experience so if there's not a way to do it using only plugins I wouldn't mind getting my hands dirty with code. So even if you don't know exactly how this can be done if appreciate it even if you pointed me in the right direction in theory only.

Thanks and have a merry Christmas, holiday, and everything else.