Hi,
You will have to create a custom component (to insert such delimiters after your XML tags in your message) and GAC it. Then, create a Send pipeline in VS project, add your custom component to the BizTalk Pipeline Components folder in the Toolbox area, drag and drop your custom component to the Pre-assemble stage in the Pipeline Designer view, and drag and drop the XML assembler to the Assemble stage (keeping its default settings for properties). The reason why you reference the component in the Pre-assemble stage is because you want to process the message before it gets serialized in the Assemble stage.
Once you have compiled and deployed your Send pipeline, use that as your Send Pipeline when configuing your port’s properties in the Administration Console.
You can refer to this MSDN article to give you an idea on how to develop such a component:
http://msdn.microsoft.com/en-us/library/aa561772(BTS.20).aspx
Also, here are a couple of blogs that detail how to develop custom components for pipelines:
http://blogs.tallan.com/biztalk/2006/11/21/custom-pipeline-components-part-1-getting-started/
Hope this helps!
Daniel.