If you're truly talking about PCI and not PCI-e, then sorry, definitely not compatible or adaptable to PCI-e x16 or any other PCI-e size. PCI is from 1990, while PCI-e is a considerably newer standard. MSI had the gall to put 2 PCI slots onto my brand new B350 board (probably because they ran out of lanes to do anything useful) and the only thing I could use those slots for was a USB 2.0 expansion card, sound card, or a slow Ethernet adapter. It's a dead standard. They should have put a PCI-e x4 in for roughly the same bandwidth load and it would have been way more useful.
This is a helpful primer to learn more about it. I guess the newest PCI is still 2x the bandwidth of PCI-e x1, so it's not total shit...