For those who might find this useful, I found that you can edit an existing game in the scenario editor. You have to copy the ".park" save file into the "Scenario" folder & rename the extension to ".scenario"

After editing & resaving it in the editor, to say add new entrances, you can copy & rename the file back & Load it like normal keeping the year, but be warrned that it will reset the level of all your employees & the maintenace time for the rides. You'll need to hire some extra mechanices for a short while.