@GayathriL

This is a bit of a hack but I've got this working by doing the following:

In your activity

1. Open find the pzInsKey of the Data-WorkAttach-File instance that contains the excel file you want to parse.

2. Write some java to call the following function, this will save a copy of the file to the temp directory on the node filesystem:

java.lang.String sendFile(java.lang.String aInstanceHandle,
                          java.lang.String aFileSourceReference,
                          boolean aIsBase64Encoded,
                          java.lang.String aFileNameReference,
                          java.lang.String aFileName,
                          boolean aPersistFileToServiceExport,
                          StringMap aHttpHeaders,
                          boolean aSendForDownload)

eg.

String strInsHandle = tools.getParamValue("InstanceHandle"); // pzInsKey of the Data-WorkAttach-File instance you want to open
String strFileSourceRef = "pyAttachStream";
Boolean boolIsBase64Encoded = true;
String strFileNameReference = tools.getParamValue("FileNameReference"); //File name, this is arbitrary but make sure you use the same when calling the parse excel activity.
Boolean boolPersistFileToServiceExport = true; // instructs the function to save file in the temp directory.
Boolean boolSendFileForDownload = false;

tools.sendFile(strInsHandle, strFileSourceRef, boolIsBase64Encoded, strFileNameReference, null, boolPersistFileToServiceExport, null, boolSendFileForDownload);

3. Set param.FSFileName = pxProcess.pxServiceExportPath + <<<your file name reference given in previous step>>>

4. Call pxParseExcelFile (or the MSOParseExcel in older versions of Pega). Remember to set param.bDeleteFile to true to ensure the temp file is deleted from the service export filepath.