I think you are a little confuse on report 
I suggest you to read the blog of @pmandal, my topic and the documentation on reports.
You can insert a script in some particular section and each section execute the code for a spefic part of the report. For example, in chart section you can execute the code to build a chart not for add columns.
But, everything starts on the source, Filter and Java source are different and you cant change the type without review the others sections.
In this case, the report is based on sailpoint.object.IdentityRequestItem object and you can managed like a column every single property on this class. If you want add something there is not in this class you can use the ExtendedColumnScript to create the new columns, but starting from the property of sailpoint.object.IdentityRequestItem class.
Otherwise, you can the filter or a hql query like source, each with own parameters.