Answer:
The best idea is to do it the same way the original will was done. If a lawyer drafted the will, have him handle any addendums. Probably the best idea is to do an entirely new will, to avoid any possibility of confusion. Its possible to make a holographic addendum to even a will drafted by a lawyer, but the same requirements would apply to a holographic addendum as would apply to a holographic will - it must be in the handwriting of the testator, written in cursive script and not printed, signed by the testator, and found among his important papers after his death.