Does anyone know if a PCI-X card can be used in a PCI-E slot?
No, i believe you can use PCI in PCI-X (but not vice versa) since PCI-X is a extension to the PCI protocol. The only slot that a PCI-X card will work with is a PCI X slot
PCI-E is a whole different distinct protocol despite the name and is electrically and physically incompatible - the card definitely won't fit by design, and almost everything is different between PCI/PCI-X and PCI-E
It almost absolutely will not work
In addition to what Journeyman Geek has rightly said.
You can still use a riser card implementation which will provide with a proper PCI-X to PCIe bridge. One such option is this