In principal you have a password option in Excel application scope that can help you work with password protected Excels. If you don’t want the Excel to be showing while the robot is working you can use the File Access Activities (those under System->File->Workbook) or Uncheck the visible property
If you want to enable protection on a Excel during the process I think the simplest way is to create a VBA Macro that locks the Excel. For that you can check out this link http://www.excel-easy.com/vba/examples/protect-macro.html.
If you want to give the password dynamically you can write inside a cell in Excel from UiPath Studio, use the Macro and then delete it.
Hope this helped let me know if your having issues.