We are adding the historical balances of one of our new subsidiaries. We have unrealized forex gains/losses as of our cutoff date. What's the best way to handle this, so that next month, we can run the currency revaluation in NS properly taking into consideration those unrealized forex gains/losses?

As part of the month end tasks, those transactions will be added to unrealised gains/loss. Then a reverse journal will be created on the first day of the next month. So you run the unrealised gain/ loss journals until the transactions are paid. So you don't need to do anything special for the new imported transactions