jOpenDocument allows you to concatenate multiple text documents.
First you load the documents that you want to concatenate. Then you use the add() method and save the resulting document (the .odt extension is automatically added).
All the sytles are properly merged.
If you want to know more, look at the package org.jopendocument.dom
The complete example is located in org/jopendocument/sample/TextCat.java