Editing the saved game by hand (with a hex editor) is not really an option as the relevant data in the saved game is compressed. It should be possible to write a savegame editor, which would have to replicate the way the savegame is loaded from the game.

It is however not very hard to edit the source to give you more time. But that means you have to recompile the source yourself. I don't know whether you have the skills and tools to do that. If not, maybe someone else could do it for you.
This is what has to be done:
- edit src/sc2code/globdata.h, and edit the line saying '#define YEARS_TO_KOHRAH_VICTORY 4', by replacing the 4 by however many years you want (excluding the extra year you get for the Utwig mission). Races in the game will think you've got this many years now, but the Kohr-Ah victory has already been scheduled, and the date is saved in your saved game, so this will only help for new games. If you want to change this date, you'll need to make the following modification (in addition to this one):
- edit src/sc2code/load.c, and insert before the line saying "UnlockEvent (hEvent);" the following lines:
               if (EventPtr->func_index == KOHR_AH_VICTORIOUS_EVENT) {
                   UnlockEvent (hEvent);
                   FreeEvent (hEvent);
                   continue;
               }
Then leave the lines after that alone that say
               UnlockEvent (hEvent);
               PutEvent (hEvent);
           }
       }
and then add:
       AddEvent (ABSOLUTE_EVENT, 2, 17, START_YEAR + YEARS_TO_KOHRAH_VICTORY,
                   KOHR_AH_VICTORIOUS_EVENT);
These first section of code added makes sure the event from the savegame is not loaded, and the second section adds the event to be triggered when you want it. (I couldn't just modify the event from the saved game, as the queue should always be sorted on date).

If you're already in the extension time this won't work; the event for the destruction of the first race is already scheduled. Something similar can be done for that, but let's see if this is enough for your purposes first.