A good example of overriding a lookup on a report dialog is on the ProjMissingHourRegUIBuilder class. In simple, you need to create a UIBuilder class that extends SrsReportDataContractUIBuilder.
You can create grouping; vertical or horizontal alignment; change the number of columns; or add look ups. The following methods can be overridden to add necessary functionality:
-  SrsReportDataContractUIBuilder.addDialogField 
-  SrsReportDataContractUIBuilder.getFromDialog 
-  SrsReportDataContractUIBuilder.build 
-  SrsReportDataContractUIBuilder.buildGroup 
Reference:
http://msdn.microsoft.com/en-us/library/srsreportdatacontractuibuilder.aspx