The same as the post above, both the RAM and the GPU run differently. The GPU is used to generate all types of media including video's, images etc... and the RAM stores information on the current programmes you a running on your computer at any given time.
The only time you can do that is if you are using a card that can use turbocache. That is only implemented within laptop gpus. They do this to cut down on costs, but as soon as the card moves over its vram memory into the system ram you will experience major slowdowns since vram is significantly faster ( ddr vs gddr).
Correct me if i'm wrong, but I have 8GB of RAM & my GPU has 1GB of Shared memory. Whenever I check my specs on websites, it says I have 4+ GBs of video memory. So, i'm just guessing if your GPU has Shared memory, it can increase it's RAM (not Physically of course).