Hello Melissa Ortega,
Adding strength to step class would be great fun and effective. Do you know what your clients goals are? You could go from there. I would add in upper body and core with bodyweight exercises which can be done with or without the step. I would cool down before doing the core work, followed by the final stretch segment to end the class.
Have you asked your clients to bring music with them that they enjoy? Whatever music gets them motivated and is not too fast; 100 beats per minute for beginners up to 129 beats per minute for advanced if you step to the beat.
Step class should already be cardio. You could do intervals by moving faster and slower, raising and lowering the step height, also adding in hops here and there.
Boxing moves are fun and cardiovascular, too; both punching and kicking.
Here are two sites you may like for fitness music choices:
I hope this helps you.
NAPS 2 B Fit