You can still use the casing, but USB 3.0 functionality won't be present.
USB 3.0 cards are
PCI-E only, there are no
PCI USB 3.0 cards.
Pls note that on older motherboards with PCI-E slot(s), the PCI-E slots are only version 1.x, not version 2.x.
If you use a USB 3.0 card on a PCI-E version 1.x motherboard, your USB 3.0 will run at only up to half its max speed (but will still be faster than USB 2.0).