I would have two globals, lvlScore (level score) ttlScore (total score)
lets say my game has a square that has the gold to touch a circle, along the way I pick up the stars, each time I pick up a star I add 1 to lvlScore. I pick up 5 stars then my square touches the circle. Now its time to add.
Event: Square is in touch of circle
Action1: Add to Variable: ttlScore Value: lvlScore
Action2: set lvlScore to 0
The add value is found under system
Action2 will allow you to use the same global for each level. Keep in mind you will need to trigger it to set to zero if you fail a level to.