In theory, it's doable. If you can convert serial to parallel, the reverse is possible. I dunno how many PCIe gfx cards would work in a single lane though. Not many, I expect. You can get GPUs designed for PCIe with AGP bridging logic to work with older v3 AGP slots, but vanilla PCI is significantly further back still.
Attempting to covert multiple lanes on the card to parallel PCI I simply cannot see working, given the way PCIe does point to point.