I would like to ask, is it possible to add a custom field into the document library creation wizzard ?
There may be some valid cases where you need to offer a different experience when provisioning document libraries or sites. If the out of the box process doesn't meet your needs, especially for a specific business case, you can always write a custom feature that can look and behave exactly the way you would like it to.
Here is the MSDN reference for the SPListCollection.Add() method. http://msdn.microsoft.com/en-us/library/ms477358.aspx