If you explicitly say:
ID INT NOT NULL
Then you cannot set it's value to null using a stored procedure or by any other means, and you have told SQL that the column can only
contain intger values, and not nulls.
The only way to do it is to change the definition of the column to allow nulls:
ID INT NULL
However, given that this is an ID column, it is probably best to leave them as NOT NULL so that you don't get duplicates...