This is a limitation of WS-BPEL. The standards compliant way to achieve this is to run an XSL tranformation. See page 65 of the WS-BPEL specification. To make this a little easier, ODE provides a couple of XPath extensions that allow for appending or inserting elements into a list. See here for a list. An example of such an assign looks as follows:
<assign>
<copy>
<from>ode:insert-after($parent, $parent/child::node(), $siblings)</from>
<to variable="parent"/>
</copy>
</assign>