usually, outlook does that if it cannot map a name from the contacts list your importing. Otherwise, it will put in the first and last name if it finds them.
You can also do a manual mapping if you want where you can set up what fields from the old list to map to into Outlook. Usually, this is done when you are importing something such as a comma-delimited (CSV) file.
If the old contact list can be exported into CSV format, you could then do the manual mapping that way.