ASP.NET Identity: Inheritance with Code First vs. Model First

Question

I have been using ASP.NET Identity Code First and I wanted to check out Model First.

One thing that I have run into: how do I add something as a base class to an entity in Model First when the base class does not exist as a table?

For instance, with code first I can extend the IdentityUser as follows:

public class User : IdentityUser
{
}

But with Model First, it is only letting me choose things as the base class if they exist as an entity in the model - so how do I go about creating a User that extends IdentityUser with Model First?