Shared video memory? What's that? Your computer has ram, your video card has ram. You can change the amount of ram in your computer. But your video card is what it is. It cannot be changed.
I think what your want is more ram for your computer. Go to OWC to see possible choices for your model. You don't have to buy there if you don't want (but I always recommend them). But at least you will see what choices are available to you.
Most iMacs come with video circuits with dedicated memory. However some use a video circuit that uses the iMac RAM for it's memory. For example, the Nvidia 9400M uses a minimum of 256MB of the iMac RAM. I'm not aware of any way to force the video circuit to use more memory for it's needs. This Apple note discusses this, and says you should have matched iMac memory to maximize performance: http://support.apple.com/kb/HT3246