Do you mean weight work in addition to the STS program?
I once tried to add a 4th weight day into my STS week. Others might be able to handle it, but I couldn't. I was exhausted.
One thing you could think about would be to add in more leg work. You could use a circuit workout premix that has only cardio and lower body weights, or any leg segment from one of Cathe's DVDs like the floor section of Pyramid Lower Body. As long as you keep the weights lower and space it some distance from your STS Leg day, that could be a nice change up in the rotation.