You can add an issue to a structure both from the Structure Board and from the Issue Page.
On the Structure Board, use Search to find the desired issues and add them to a structure using drag-and-drop, copy/paste, or the Add button on the search panel.
When searching, make sure the JIRA Search Results secondary panel is switched on. Use More Issues button on the search panel.
On an Issue Page, if the issue you are viewing is not in the current structure, the Structure section contains Current Issue secondary panel and you can drag the issue to the current structure from there. You can also select a different structure using Structure Selector. Besides, you can use Search on the issue page as well and add other issues to the structure.
Adding issues to a structure can be undone.