After an app is published to a stream, the approved sheets (or the sheets in the application at publish time) are locked from editing and cannot be changed.
If you want to change the approved sheets, you need to go into the QMC, duplicate the app, and edit the duplicate. After you make your changes, you can replace the published app with the copy.
The only way to make a sheet you create an approved sheet is to make a copy of the app in the QMC and make yourself the owner. Then you can change the "approved" sheets. Afterward, you republish the app to the stream.
Like I said, you cannot add a sheet to the "Approved" sheets, nor can you edit an "Approved" sheet in place on an application published to a stream.
If you are trying to duplicate a sheet in a published app that is possible. The sheet will go into the My Sheets section and then you can edit it. Once you complete editing, you can "publish" the sheet for the community to see, but it will not be added to the "Approved" sheets. It will be added to the "Published by me" section of the app, and other users will see it in a "Community" section.
All of this behavior is governed by Security Rules. However, the publishing of apps to streams will turn off direct editing of "Approved" sheets regardless of security rules.