How users can retweet from a different app and it will reflect on the twitter feed

I have an app that allow people to share quotes they like on the app and it automatically reflect on their timeline.

The users are authenticated from the app with twitter auth and i have their UserAccessToken and UserAccessTokenSecret, but because i do not have TwitterConsumerKey and TwitterCOnsumerSecret, i am unable to post to their timeline.

I need guidiance on how i can post to post or retweet(existing twitter post that has valid twitter_post_id) on their timeline.