MergeContent by itself doesn't allow you to merge parts of the FlowFiles together. However, an option that you can use is to put RouteText before MergeContent. In RouteText, set the Matching Strategy to "Satisfies Expression." Then, add a property named "header" with a value of "${lineNo:equals(1)}". Leave all others as default values. This will route the first line of each FlowFile to the "header" relationship.

You can then auto-terminate the "header" relationship and route "unmatched" to MergeContent. That is, filter out the header line and route everything else to Merge Content.

With MergeContent, you can then set "Header" to the text of your header and set "Delimiter Strategy" to "Text." This will cause MergeContent to add that header line back to the merged FlowFile for you.

Sorry that this is so non-trivial, but I think this approach will at least give you what you're looking for.