MultiSelect select box

Is there a way to populate the multiselect box on the client and then have those values bound back to the XML.

We've got the multiselect box to populate based on firing onlcik from a button but the values are not binding back.

The first thing to check are the Data Binding settings for the "Multi-selectbox" on the Page Bindings Tab. The default is "CSV String", but there is also an option for "XML Structure". The former creates a comma separated list of values into a single element.
The "XML Structure" option will create one or more elements of the same name for the data options selected.

Check that the Data Binding instance document on the right hand panel of the screen has at least an empty element for the one you are trying to populate. With the XML Structure it would create more elements if values were selected.