I work for CodeSmith Tools.. This currently isn't possible, but I've added this to the backlog. It would be cool if you could specify a source file with the content like you can do with connection strings in an app.config.
I've really wanted to move csla to use an xml file for entity generation. However, a lot of people don't want to maintain this xml file :(. This would however, solve this issue. You could always use the copy to clipboard feature in the property grid and then paste the properties to the other outputs in the csp. This would help keep these files in sync.