In my case, I had several content types that had been created on a project site and it was decided to they needed to be available for all projects. I used the ShareGate application (Copy Content and Structure) to copy the needed content types from the source project site to the internal SharePoint online content type hub (https://your tenant/sites/contenttypehub/). I'm sure this could also be accomplished using PowerShell. When the copy was complete I published the content types through SharePoint Admin > Content Services > Content Type Gallery. After publication, I was able to add the new content type using the add column menu in my library.