I'm new to Squiffy, and I'm wondering if there's any way to implement event rarity to a game?

For example:
Say, there's a "continue" option. When the player clicks it, the computer randomizes a number (say, 1-10)
If the generator spits out a number greater than 5, the player gets x event.
If the generator spits out a number greater than 2, but less than 5, the player gets y event.
If the generator spits out 1, the player gets z event.

Is there any way to do this?

Yep, using javascript Math.random() method at the begining of the new section.