There is a script that analyzes the import file and determines it's correct to import it into the database.  The script would have to be updated to include the new field most likely.  You may need to talk to your X3 partner.

Assuming you created a database field as well, because if you didn't do that the import would have nowhere to store the data.... On the object did you add your new screen which contains the new fields to the Import block? If so, it should work with the default script in my experience. On 7 - 11 anyway. If you named the screen fields different from the table fields, all bets are off. 

This seems to work for me when the table field name and screen field name differ.

In the screen dictionary add a action for the problem field.

Type: Initialisation

Action: SPE

Execution: Import/batch

# Goes in the SPEXXX.src file defined in the "Specific script" field in the Screen Dictionary
# Name of screen field: "YFLD"
# Name of database field: "XFLD"
# Abbreviation for database table: "ZABR"
Subprog D_YFLD(VALEUR)
Variable Char    VALEUR()
  VALEUR = [F:ZABR]XFLD
End