By having to faff around with the code, as it seems each area seems to be coded differently. Generally though, there's a integer that increments how many choices there are, and then a selection that picks a random encounter up to that integers value. All of this will be in the areas classes/classes/scenes/areas/***.as file.
In bog.as for example, chooser is the number of scenes. It's currently 5, so you'll want to up this to 6.
Then, you'd want to add an else if statement in the same function after the last else if already there, but before else (which acts as a just in case).
//the double equals are important. And the random chooser will work from 0 to 5, so 6 will never be an option
else if (chooser == 5) {
//this code you can copy from exploration.as where goblin encounters normally are
kGAMECLASS.goblinShamanScene.goblinShamanEncounter();
spriteSelect(124);
return;
}
Note: In theory, this code works