I'd advise against tampering with a prebuilt system since it will void your warranty and the parts within that chassis are proprietary which means that the system that came shipped as is is meant to be that way. If you'd like to add on a GTX750Ti then you better do it on a system that can accept a standard SFX(-L)/TFX/ATX sized 450W PSU that is of reliable quality and is branded.
Your best course of action would be to build a system of your own instead of transplanting it to have that GTX750Ti in there as well.
One of my teachers while I was studying to be an Architect, said:
Why create a problem and try to solve it when you should really be solving problems that it came with?
Case in point, your trying to make the system do something it's not built to do.