I am working on a project where i have to store the data into an excel workbook, according to worksheets.
i want to have a provision where in i can save data to a single workbook but i can select the worksheet name in which i want to save the data.
The main problem is, i want to do this using MS Office Report express VI.
I agree that wanting to create an Excel Workbook using an Express VI is the "main problem". Learning to use the Report Generation Toolkit is not so difficult. There is a pretty good demonstration of how to create a Report with various types of entries, including how to "save the data to a single workbook" (it doesn't include the option to name the Worksheet, but if you get everything else working, you'll either be able to figure out how to do the renaming yourself or you can come here and we'll help you).
To find this demo, go to the Search Bar on the first page of this forum and type "Revised Excel Example". You haven't told us what version of LabVIEW you are using, but this was written in LabVIEW 2013, and one of the functions, "New Report", is now called "Create Report" (the old function will, I think, still work, but replacing this with "Create Report" will get rid of the red "X" shown here ...)