The short answer is "No". If the file is not inside RH it acts just like any other file link. If someone has accessed the file someone else will get the standard MS Office dialog. You could try creating a PDF or "Read Only" version and linking to that but you'd have the problem of remembering to creating these after each update.
You may consider using a hard path in your RoboHelp project, like: "http://myintranet/files/excelfile.xls" or "file:///H:/document/excel.xls"
The hyperlink will then allways open the file in that path. Since the file is not included, everyone who wants to open it must have acces to those directorys. When you also link to a file, every user has to have the same letter mapped to the drive where the excel is stored.
This one has a fairly simple solution. You just need to set up your Excel workbook for sharing. on the menu, select Tools > Share Workbook. Select Allow changes by more than one user... (If you don't actually want users to make changes, you can always protect the workbook (Tools > Protection > Protect shared workbook.)