How do I define a database index in HoboFields?

With HoboFields, I can declare fields for my model in its own file, like this:

class User < ActiveRecord::Base

  fields do
    login :string
    persistence_token :string
  end

end

I'd like to know if there's equivalent syntax for the add_index command I can use in vanilla Rails migrations.

class User < ActiveRecord::Base 

  fields do 
    login :string 
    persistence_token :string, :index => true
  end 

end