Installing a wireless motion sensor and glass break sensor to my wireless ADT, how?

I just signed up for 3 years contract with ADT about a month ago. I have a basic package (2 doors and 1 motion detection for $99). Now we pay $45/monthly service. I want to put 1 more motion sensor and 2 glass break sensor, but their price is ridiculously high. ADT wants to charge me an additional $240 plus tax for motion sensor and $220 for each glass break sensor. I saw other web site which sell the sensor at a fraction of ADT's prices. If I buy the sensor detector at other vendor, how would I add them on to my current wireless GE Simon XT system. Can I install this myself, or do I have to let ADT to do it? Does anyone know how to install the sensors? Also, do I need to let ADT know where I install the sensor so they'd know which zone in the house when alarm goes off. Please help. Thanks in advance.