With the above suggested method, can you not use any standard field in any tab?
What is the information you wish to provide in that field? (I mean a Date/ Time or text or long text).
And If you have already studied all this and you want to go with the method you mentioned, I think you need to check out with your abaper for any user exit/ screen exit for this transactions.
As of now, I dont have any idea about user exit/ screen exit for this but will revert you once I am having some answer.
Please do post if you are coming to some conclusion so that other members are also benefitted.
Chin2.