Hi,
I'm not sure that there is an out of the box support for impersonate users in MVC, but if you followed documentation for implementing login features, you can do a simple workaround.
Instead of using: SignInManager.PasswordSignIn(username, password, false, false);
you can sign in users without knowing password with this peace of code:
User user = UserManager.FindByName(model.UserName);
SignInManager.SignIn(user, true, true);
This will allow you to log in as 'impersonated' user into portal on MVC side. This feature should be enabled only for users with admin privilege level, so keep it in mind to restrict it on the site.
Best regards,
Dragoljub