It will NOT be possible to add extra games/pledges at Black Friday rates. The Black Friday rates are limited for a reason - they are meant for the people who jump on in the first day. Those prices will disappear later this morning.
You can add an extra $250 to your pledge for an extra copy of the core game, and you may be able to do that later on in whatever pledge manager they use if you don't want to add it now. But plan on the full price, not the Black Friday price.
Keep in mind, even without the BF price you are getting a great deal over retail rates through the KS.