@person = Person.new(params["person"])
@person.id = params["person"]["id"]
@person.save

OR… you can have the form call your ID param something else, like params["person"]["legacy_id"] and then define :legacy_id= for person:

class Person < ActiveRecord::Base
def legacy_id=(v)
self.id = v
end
# for consistency
def legacy_id
self.id
end
end

@person = Person.new(params["person"])
@person.save

Plenty of ways to hit the same solution.