Splunk exports as CSV, JSON and XML. None of those are Excel, they're only "regular non-XML data files" that get "interpreted" by Excel as data it can import.
As such, it's just comma separated values (== CSV) and there's no mechanism at all for doing formatting.
BUT there may be an answer anyway!
Have you thought of using the Splunk ODBC driver? With it, you can create a saved report in Splunk that you can retrieve at will via the ODBC driver, and if I remember it correctly formatting applied in Excel to a sheet that has as its base an ODBC data connection is retained when the data is refreshed. So, install that driver, build a report to do what you want, then tell Excel to use that as a data source!