Membership - access levels that are not subscriptions.

Hi folks!

OK - tricky one.

on this site we have a membership package you can buy.

We need to allow people to sign up for a package, let's say
"Peanut Eaters".
However, if someone is a medical expert, we want to administratively add access to content that refer specifically to medical information about peanuts.

So it's a whole different access level, but it's not for the public to sign up for it? That makes sense, yes?

How would i work that?

Thanks for your time in helping!