Nulls in program described file

I may also have this issue in a task I've been given.  I have to write a
routine that will work on any file (OVRDBF) and I'll get the field
description from the QUSLFLD API.  I was not specifically asked to have
it support NULL, but I was going to if possible.  

In a nutshell the program will take data from an ASCII file and impose
fields on it and write it out to a DB/2 file.  (The DB/2 file will be
externally described.)  Reading the FFD will tell the program what type
of format the field will need to be: CHAR, PACK/ZONE/<FLOAT?>, DATE,
TIME, TIMESTAMP, HEX, BIN, etc.  The record will be built as a single
char field, so I will need to write the numerics out in hexadecimal
characters.  I was planning on having NULL capable fields BE null if
they are not in the ASCII file, but if I have to I can fall back to a
default value approach.  (I am also going to use the default value from
the FFD for fields that are not in the ASCII file if one is specified in
the DDS.)