I agree with the theory that people just trade one addiction for another so how can I get addicted to exercise?

By exercising. Exercise tends to be self perpetuating....The more you do it, the better you feel, and the more you want to exercise. Further, when you've established the exercise habit (not addiction), then on days you don't exercise you will notice you don't feel as well, which will make you want to refrain from missing exercise.
Silly question. You really don't want to be "addicted" to anything. Too much of even exercise isn't good. I get the point of your question, but there's nothing good about having an addiction. For that reason, i find it to be a silly question.