If you search for "Creating and editing section profile data" in the Help file it outlines the XML format and requirements for the delivered section files. As long as you're just adding new section sizes to existing section shapes, it's a matter of copying and pasting blocks of text and changing the values in the pasted version.
Also, please see the discussion Add custom beam sections to Structural Modeling.
You could also file a Service Request asking us to add Thai structural sizes to the NM Dataset.