Unfortunately, no truly (native) FlowHeater XML Adapter will be developed until version 2.0. The problem with XML is all the substructures possible and FlowHeater can presently not cope with them or it would become really complex.
If it for the simple output of a flat XML structure, I can imagine being able to achieve it with a modified DataTable Adapter. For the next version I will publish a further example of how (simple) XML exports are possible.
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I have published an example for you of how you can perform exports in XML format with the DataTable Adapter in FlowHeater. The example is now available as a Definition in the ZIP download of FlowHeater.