Move the folder Library/Application Support/Armory/databases to your extra drive. I will assume you place it in /Volumes/Extra/Armory/databases
Now open a Terminal, and make the softlink like this (the bold is what you type, the non-bold is responses)
cd 'Library/Application Support/Armory'
ls databases
ls: databases: No such file or directory
ln -s /Volumes/Extra/Armory/databases
ls databases
blocks blocks-lock headers headers-lock history history-lock txhints txhints-lock
Note how the first time you do ls databases you get an error. That is because you have moved the folder away. If you do not get an error you have not moved it (did you perhaps copy it instead) and need to move/remove it first.
The second time you see the contents of the databases folder on the other drive, you have just placed a "softlink" to the folder where Armory expect the folder itself. Unlike Mac OS aliases and Windows shortcuts, the application does not need to be programmed to follow a softlink, it just happens automatically (by the operating system).
You can of course do the similar thing for the Bitcoin Core database. Or you can softlink the entire Library/Application Support/Armory folder instead of just Library/Application Support/Armory/databases, as I have done.
This seemed to do the trick. Thanks!