Bear in mind, you are adding nearly 50 lbs to a machine that, more than likely, has been engineered specifically for the original weight stack. Companies do not tend to "over-engineer" products. They need to keep spacial considerations in mind and keep material and freight cost to a minimum.
If this is so you will likely run into issues later on with, at the very least, the pulleys and I would image the weight stack support tube.
There were certainly safety factors included in the design of the machine, however I would not make too many assumptions if I were you. My point with all of this being, if you are successful, you will get some life out of the machine but just be careful, you could loose an eye or something.
What I mean by if you are successful...have fun drilling holes in the stack bar extension, and how you will weld an extension on to the existing stack bar and have no failure or interference issues...geez!