Your computer already do this automatically. VRAM is only storing very frequently-used resources that need to be accessed in a very short amount of time. If/when the VRAM is used up, most computers will attempt to transfer some of the less important information your your RAM.
DirectX, AGP and PCI-Express cards do this automatically.
You could manually assign more RAM to be used exclusively by your GPU, but this requires a lot of internal knowledge about computers and you have to know exactly what you're doing, because you can really mess up your computer this way. I won't add links to any tools that can do this, because there are always people stupid enough to attempt this without proper knowledge (Not saying you are, but there are bound to be someone in the future at least).
If you really want to do this manually, research it until you can feel comfortable messing with the RAM, an find the tools yourself. I don't want to be responsible for you frying your RAM cards.
If you do not have the proper knowledge about how to do this (which very few computer users do), you're better off inserting/buying a new GPU with more VRAM.
If you're on a laptop, chances are that your internal GPU uses some of your RAM if it runs out of VRAM. If this is the case, you should upgrade your laptop's RAM capacity.
RAM is a lot slower than VRAM, so you'll notice some real slowdowns in the rendering times. Also, again, do not attempt manually assigning RAM to your GPU without proper knowledge about this.
Hope this helps.