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I would say, it is not possible. What would be possible: Use one single column and add a component in it, displaying the wanted result. The downside of this approach is, that you loose the ability to sort the table.
Don't know what is possible with the TreeTable as I didn't try it out yet, but a custom "row-component" is pretty easy. See Book of Vaadin: Table, especially section 5.14.2 "Components inside a Table" as a starting-point.