Null default value for SQL insert

Hello guys

Can you help with the following:

A long preface:
We use a Web based home grown system to upload many files to our i machine
(V7R1)
using SQL Insert.
When Inserting a record to a file using SQL Insert with partial list of
fields
(as in File having fields A, B, C and the insert is "Insert into File (A,B)
Values('1','2')"
the the remaining fields are populated with default values (field C in the
example above).

Numeric fields are populated with zeroes,
Alphanumeric fields with blanks
Date fields with Current system date
Time fields with Current system time
etc.

Default values for fields can be defiled using DDS DFT('xxx')

Now for the question:
How do you define a *Null default value* with the A/M DDS DFT ?
(and no, adding the missing fields to the Insert staement in no option)

Thanks
Gad