File Import: Mapping one source field to multiple existing fields

It's not possible.  One alternative  would be to import to a temporary staging table.  Then set up a saved table to table copy to copy the records from the staging table to the real table.  In the saved T2T copy, you can map a source fields into multiple target fields.

I have done this for clients, I set up an admin import record in a special table that only has one record. 
I put a button there to clear the scratchpad table.
Another button to lead the user to the file import screen where you upload the data
And then another button to copy the records from the scratchpad table into the real table.  

I show summary totals of how many records are on the Scratchpad and how many records were imported today.

So that really makes the import process easy for front line staff so that the import process can be more easily delegated to a more clerical level user.