XML Datasource - change field type

Hi,

I am using XML datasource for my reporting.
I create a new datasource and write a xquery to fetch data - which I can preview.
The default type for the data is 'String' - and i want to change the data type for different measures.
The query builder does not provide any way to do this, how can i change the data type after forming my queries?
I am very new to Pentaho reporting - so any help will be appreciated.

Thanks,
Vash