How to add a feature to an existing Rails project

I have recently downloaded a new project(open source),i found certain
features missing like blog,forum ,chat etc… …so I like to add those
features to the project .My problem if run 'rails forum 'it will create
new rails project but i want to add to the existing project. I have
business logic. . . .I had created models ruby script/generate model forum ruby script/generate model topic ruby script/generate model post rake db:migrate ruby script/generate migration add_foreign_to_topics forum_id:integer ruby script/generate migration add_foreign_to_post topic_id:integer rake db:migrate;
Then i ran 'ruby script/generate controller forum ,it was asking should
overwrite or not,so i am stuck up here,i need to create a controller and
view for this feature.I am following this tutorial

and i have already user table etc…