Yes, you can add existing User Stories in a Release by 2 ways:Using Planning Board:
Open the panel type (Backlog or Sprint Panel) where you have this User Story. Open Release Backlog in another panel. Locate the User Story. Drag and drop User Stories to add them in selected Release.Using Release Board:
You can add an existing User Story by using drop down > User Story – existing. It will open Planning Board where you can search User Story. You can open different panels on this board. Open the Sprint/Backlog in one panel and Release in another panel. You can drag and drop selected Items to Release Panel.