Boy, I've never heard of anything like that, but if it exists, I think it wouldn't work all that well. The PCI bus runs at 1/3 to 1/4 the speed of your FSB depending so the memory access would really be crippled. I would think the system would have a major hard time synchronizing the "fast" memory with the "slow" pci memory. Might hurt overall performance.
Why not just replace 1 or more of your old DIMMS with bigger ones?