Is there a way to make a body field automatically become the first comment on a node?
Without coding, I'd go for the module Rules :
You can create action that are triggered after certain events, under certain conditions.
In your case you can try to create a rule like this one :


Event : When a Node is Created
Conditions : You probably want to filter the node type maybe ?
Action : Create an entity

Select Comment as your entity type
Node referencing the comment : Use the available token "node" (the newly created node that triggered your rule)
Comment body : Use the available token node:body



And you're done .. 0 code, 1 module, 2 mins to setup your rule :-)