I'm not sure it this works in C2, but everytime you add score, I would try a comparison between two values, score%10 and 0, if true add a life. score%10 returns the rest of the equation, so if score is a multiple of 10 it adds a life.
blackhornet, this solution would end up giving the player, 1 life, then 2, then 3 and so on if he doesn't subtract 10 from score.
You can also use blackhornet's solution together with a 'hidden score', everytime it reachs 10, add a life and subtract 10 from it.