Yes a NVIDIA card will work on a crossfire board. PCI-E X16 slots are all the same.

Can you CrossFire AMD and Nvidia cards?

This configuration enables you to use AMD and Nvidia video cards in one PC, and you don't need cumbersome setups like AMD's CrossFire or Nvidia's SLI (which link multiple cards together — but only of the corresponding manufacturer, and the cards must be identical). You just plug into your existing PCIe ports.

Does CrossFire work with different cards?

CrossFire can be implemented with GPU cards of the same generation, and manufacture (this is compared to Nvidia's SLI, which generally only works if all cards have the same GPU, and brand). ... However, GPUs not in the same hundred series cannot be crossfired successfully (e.g. a 5770 cannot run with a 5870).