Just moved to Netsuite and realized our beginning inventory balance was off. Does anyone know how I would add serialized inventory to my December 2020 ending balance?

In short, you'll need to use an inventory adjustment in that period. However, before you proceed you should first understand the root cause, have the list of missing serialized inventory, and ensure existing serialized inventory matches what is in the system. If the period is closed and you do not want to impact the financials after opening it again, then you'll also probably want to create a reversing journal entry to reverse the impact of any new/updated inventory adjustment and move it to the current period.