Down-casting due to custom Datasources and Preview

I am using a custom datasource that implements some specific methods that are called from with the jasper .jrxml templates to fill subreports. To invoke these methods I need top downcast the REPORT_DATA_SOURCE parameter resulting in an expression like

This all works very nicely - unless I want to invoke the Preview capailities of Jasperstudio. Necessarily it fails as the downcast requires the presence of the java library. So I wonder if I can configure my Jasperstudio in such a way that my custom libraries will get resolved?

That said, I wonder if I can somehow specify within Jasperstudio the file that underlies my custom datasource? I know that this is possible for XML datasources.