Hi,
Can you provide more details(screenshot would be better) about your requirement? It would make others easier to find a solution for you.
Suppose that you might want to populate a field with the value of the Created By field automatically after an item is created.
If this is the case, a suggestion is that you can create a custom Event Receiver for your scenario.
Here is a step by step sample on creating a simple Item added event receiver for Custom List:
http://msdn.microsoft.com/en-us/library/ff398052.aspx
More information on Event Receiver for your reference:
http://msdn.microsoft.com/en-us/library/gg749858(v=office.14).aspx
http://msdn.microsoft.com/en-us/library/ff408183(v=office.14).aspx
Thanks
Patrick Liang