draw.io for Confluence Cloud allows users to add custom libraries (custom shape sets) to their editor, once those libraries are added to the Confluence instance.

To populate the list of available libraries in the dialog for File, Open library from, Confluence Cloud create a space with the key DRAWIOCONFIG, create a page with the title "Libraries" in that space and add the library files as attachments.

You can create the library files using File, New Library, save them to your device and attach them to the Libraries page from there:

The title of the library is taken from the attachment name.