Hello! I'm glad to hear that you and your fiancé have been approved for your NOA2.
Regarding your first question, it is unfortunately not possible to add a family member to an existing K1 visa application. You would need to start the process again from the beginning if you wish to bring your fiancé's son with her.
As for the second question, the best way to handle this situation would be to petition for your fiancé's son as an immediate family member once you are married. You can do this by filing a Form I-130, Petition for Alien Relative, and if he is already your stepson, you can also consider filing for his adoption as part of the process.
It is important to note that processing times for immigration applications can vary and are subject to change. You may check the USCIS processing times for an estimate of how long the process may take.
I hope this information helps. Good luck with your journey!
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