Add the new forked coin as 'gift' or 'income' (depending on the tax laws of your country).
Since the original coin retains the cost base, the forked coin will need to be entered with a costbase of 0.
However, an absolute asset value of 0 will cause the software to search for and possibly enter an asset value of the day automatically. Therefore please enter the value of 0.00000001 FIAT. This will prevent overwriting and set a value of 0 for all calculations and tax report purposes.Ignore the resulting warning and add the transaction.