There is a good hint: when defining a statement for a test system (or, possibly, also in the pop-up window), the "Declare" button appears above the XPath-Expression field. Actually, it doesn’t look like a button until you point the mouse at it, so I didn’t understand it at first.
Just click the SoapUI button (I actually use 5.2.1) will add declarations for you that you can use.
I found this function by coincidence, since it is not visible. Perhaps this will help too ...
Jake
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