Unless you have a shared integrated gpu, or the gpu supports ati's hypermemory/nvidia's turbocache, you cannot just add system ram to a gpu.
They are separate entities, with their own ram. Only exceptions are the ones I mentioned above, which were designed with the capability to use system ram.
Another thing, such an idea is very pointless. Increasing a gpu's ram gives very little performance increase, and even less if this ram is shared system ram. Not only that, you would be taking away from the available ram.