Can I send flatfile string to a custom pipeline within an orchestration?

Hi, I want to send a string containing flat file data to a custom pipeline with a flat file dissasamble stage. I can't get it to work to use the ExecuteReceivePipeline method from an expression shape in my orchestration. I've tried to create a non serialized string type for the input message to call the pipeline, but then I get an exception that tha data can't be recognized. Anyone have any idea on how I can continue or a better way to do what I'm trying to do. I would really apprechiate the help. Thanks!