Create a read only excel file from SSIS?
That's a filesystem concern, not an SSIS one. Any file that SSIS creates will be owned by the user SSIS runs as. The best solution would be to set the ACL on the target folder to be what you want; when SSIS creates the file it will get those permissions by default.

If you can't do that for some operational reason, you can manipulate the file after creation with a command task. You can use SetACL or any command line tools for this.