You can use Read range and read the complete excel file and use for each and in for each use datatable.columns and arument as system.data.datacolumns and inside foreach use assign and use item.columnname.tostring and append.
Refer the below workflow and the sample excel sheet, so the below workflow will give you all the column names in a single variable separated by space.
Thanks a lot. It helped. Just one question though. We are appending the every column name with previous one. So again we need some method to separate those names using suitable delimiter(say comma","). How do i do that.