Call a block on a Model

Instead of defining a scope in a class like this:

 scope :first_user, -> { first }

And calling it like this: User.first_user

I would like to define a block in another class, that can be called on the user class and works like a Scope:

This code is not working but it should signalize what behaviour I want to achieve:

class Manage

 def get_first_user
    User.&first_added
 end

 def first_added
   Proc.new { first }
 end
end 

When I run this code:

a = Manage.new
a.get_first_user

it says me, & undefined method for User. How can I execute the Block defined in first_added on the User model?

How can I in general call a block on a class? Thanks