Hi Gee,
I've got two address books in Outlook (under a 365 account), when I wanted to combine them I just went to the "My Contacts" page, selected all of the contacts from the "old" account (using CTRL+A to highlight them all) and then dragged them to the new account on the left hand menu. Basically a drag-and-drop to move them
.
I've just tested it again now using a test contact and it works great. If you need any more detail, just let me know and I'll attach screenshots.
Cheers,
Ian