Can I use the "Secretary Problem" to find the worst candidate, too? As you know, we use the "Secretary Problem" to choose the single best candidate. Now I would like to know can we use this rule to find the worst candidate, too? If yes, how to accomplish this?

Yes.

The algorithm simply finds the participant who is **_optimal_** according to some property, but does not care what that property **_means_** (as long as it defines an ordering relation on the candidates). Changing that property from **_being good_** to **_being bad_** (or whatever else) is just a definition without algorithmic relevance.