You don't even need to do any customization or programming to change this list. By adding the screen to Automation Steps screen, you can put the Source field in the Fields tab of the automation definition and override the combo box values. Please note that if you're trying that with Acumatica 5.0, you may need to remove the "Opportunities" list as entry point from the generic inquiries, otherwise it will interfere with your selection when you try to select the Opportunities screen from the Automation Steps.
If you want to do it via programming - you'd need to replace the CRMSourcesAttribute on the field with your own version of this attribute. This attribute is fairly simple and is only derived from PXStringList attribute:
public class CRMSourcesAttribute : PXStringListAttribute
{
public const string _WEB = "W";
public const string _PHONE_INQ = "H";
public const string _REFERRAL = "R";
public const string _PURCHASED_LIST = "L";
public const string _OTHER = "O";
public CRMSourcesAttribute() :
base(new[] { _WEB, _PHONE_INQ, _REFERRAL, _PURCHASED_LIST, _OTHER },
new[] { Messages.Web, Messages.PhoneInq, Messages.Referral, Messages.PurchasedList, Messages.Other })
{
}
}