Since ATMOS is indistinguishable from TrueHD to MC, you can take one of two approaches:
1) Bitstream ALL TrueHD, including ATMOS. This would require a rule that is basically:
If [Compression] field contains TrueHD, use Zone 1.
2) Bitstream ONLY ATMOS:
To do this, you will need to create a custom field called something like Audio CODEC. This is what I do.
You will have to fill in this manually for your ATMOS files, with, say, ATMOS.
Then the rule is basically:
If [Audio CODEC] field is ATMOS, use Zone 1.
(I auto-populate the [Audio CODEC] tag for most of my files upon import, by having them look at the [compression] field and seeing what is registered there.)
I have only outlined what you want to do conceptually. I have not included details of how to create a custom field in your library or how to create the zoneswitch rule. I am not on a PC with MC on it right now to help with that, and in any case you may know how to do those things already.