I would be nice to be able to add a "reply-to" field to the email template. Today this is solved by using the "From" field with two addresses separated by comma (+ custom actor).
Is it possible to extend the EmailTemplateActorModel? If yes, how?
You can define your own model which may contain any fields you want. But you need to create a custom actor for using those fields and send message (or do other things) yourself. Please see here as a sample https://github.com/episerver/EPiServer.Forms.Demo/tree/master/Implementation/Actors