SQL Server 2008 :: Update Null Enabled Field Without Interfering With Rest Of INSERT / UPDATE

If I have a table with 1 or more Nullable fields and I want to make sure that when an INSERT or UPDATE occurs and one or more of these fields are left to NULL either explicitly or implicitly is there I can set these to non-null values without interfering with the INSERT or UPDATE in as far as the other fields in the table?

EXAMPLE:

CREATE TABLE dbo.MYTABLE(
ID NUMERIC(18,0) IDENTITY(1,1) NOT NULL,
FirstName VARCHAR(50) NULL,
LastName VARCHAR(50) NULL,

[Code] ....

If an INSERT looks like any of the following what can I do to change the NULL being assigned to DateAdded to a real date, preferable the value of GetDate() at the time of the insert? I've heard of INSTEAD of Triggers but I'm not trying tto over rise the entire INSERT or update just the on (maybe 2) fields that are being left as null or explicitly set to null. The same would apply for any UPDATE where DateModified is not specified or explicitly set to NULL. I would want to change it so that DateModified is not null on any UPDATE.

INSERT INTO dbo.MYTABLE( FirstName, LastName, DateAdded)
VALUES('John','Smith',NULL)

INSERT INTO dbo.MYTABLE( FirstName, LastName)
VALUES('John','Smith')

INSERT INTO dbo.MYTABLE( FirstName, LastName, DateAdded)
SELECT FirstName, LastName, NULL
FROM MYOTHERTABLE