For a long time, I was under the impression that you couldn’t transfer items between catalogs easily. However, you can and it’s pretty easy to do.
In the Spec Editor application, navigate to the Catalog Editor tab. Believe or not I missed the first step all those times. I think that typically I tried to copy from the Spec Editor tab instead of the Catalog Editor tab.
In the catalog to copy from (source), select your items from the part list, right-click and select copy. You can use Shift or Ctrl to select multiple items.
Switch to the catalog you want to copy to.
Right-click in the part list and select Paste.