Вы используете jaxb?
Есть ли в сгенерированном коде что-то подобное?
<code>/**
* Gets the value of the parameter property.
*
* <p>
* This accessor method returns a reference to the live list,
* not a snapshot. Therefore any modification you make to the
* returned list will be present inside the JAXB object.
* This is why there is not a <CODE>set</CODE> method for the parameter property.
*
* <p>
* For example, to add a new item, do as follows:
* <pre>
* getParameter().add(newItem);
*