Как Вы добавляете некоторые шаги к capistrano задаче?

Rather than redefining deploy:migrate, you should define a before or after hook for it. First, create a new task that does the stuff you need to do:

task :custom_name do
  # whatever you need to do
end

And then set this new task to be run before or after the deploy:migrate task by doing one of the following:

before "deploy:migrate", :custom_name
after "deploy:migrate", :custom_name