Numeric data type is lost after exporting to Excel

We are developing an ERP application and for the Reporting component we are using open source BIRT report engine & open source BIRT designer tool; however we are facing a miss functionality after exporting an excel formatted file that includes numeric data type. The problem arise after exporting an excel sheet as the numeric data type is changed unexpectedly to a text data type. This prevents us from making needed business customization on the exported excel sheet. PLS help!.