There is no way to physically add more vram for your system. Depending on the specific chipset and bios used, it may be possible to allot system ram to the graphics chip. This would be done in the bios. If said setting does actually exist on your specific chipset/bios combination. If that doesn't happen, you are pretty much out of luck.
As I said though even if you did manage to get it working performance would be horrifically bad.