OK... I'm answering my own question. Maybe someone else will find this useful.
I was told by support that I couldn't wildcard searches like I asked above. It turns out you can... but not at the root of the share. In other words, if you're scanning \\server\share you can't wildcard a*,b*,etc.
But you CAN do it if you're working with a subdirectory.
Say your node is "server" , your share is "F$", you've got a subdirectory named "home", and user directories under that. You can then scan the share and add criteria for "directory pattern contains home/a*