Not supported. If you go (in the mobile dashboard app) to the "Switches" page you'll see a little "+" in the upper right and if you click that you'll then see the barcode icon to add a new switch by scanning another switches barcode to whichever switch network you're in at that time. Same goes for wireless APs. But MX networks only have a single MX in them (unless you're running warm spare HA then there's two) but still only a single "active" MX in any given network, so no "+" sign with a barcode tool behind it. In the place where you might expect the "+" sign it'll actually be a pencil icon to change its name or add tags.
What you might want to look at is the Bulk Network Configuration tool under the Organization tab. You can choose your source configuration if you already have an MX network or template defined, and have your 200 MX65W appliances listed out in a CSV file along with their names, networks, serial numbers, addressing info, etc. More on that here: https://documentation.meraki.com/zGeneral_Administration/Templates_and_Config_Sync/Using_the_Bulk_Ne...
You can also then leverage configuration templates to make mass changes to those MX65Ws as a group. More on that here https://documentation.meraki.com/zGeneral_Administration/Templates_and_Config_Sync/Managing_Multiple...
And if it's simply 200+ additions to your Dashboard inventory, you should be able to do that with the claim button on the Organization > Inventory page using one or more order numbers or serial numbers.
