It is a much better value to add park hopper to a multi-day ticket. I just went to the site to compare and to upgrade a one-day Magic Kingdom ticket to park hopper was $67. To upgrade a 4 day ticket was $90. That's only $23 to go from one day of hopping to all 4. Also, as Craig mentioned, it is not possible to only add hopping for one day - you'd need to buy a separate ticket which would cost WAY more.
It is a much better value to add park hopper to a multi-day ticket. I just went to the site to compare and to upgrade a one-day Magic Kingdom ticket to park hopper was $67. To upgrade a 4 day ticket was $90. That's only $23 to go from one day of hopping to all 4. Also, as Craig mentioned, it is not possible to only add hopping for one day - you'd need to buy a separate ticket which would cost WAY more.