Altering table to add foreign key

I have 5 migrations file which have content to create tables in my database and it worked fine. Now I want to alter each table to add foreign keys. How shall i do it? Do I need to create a new migration file for each of these new changes? or shall I modify the create_xxx_xxx.php file? When I attempted latter the migration tool was saying there were no migrations.

Please advice.

via Chebli Mohamed