Can I install an IDE (PATA) optical drive in a 2009 Mac Pro?
Short answer:

As long as you have only one optical drive installed in your MacPro, you have a full 5.25" bay either under or above it available to stuff something into. I have a pair of SSDs installed in my lower optical bay.

Shorter answer:

If the IDE-SATA adapter is smaller than the optical drive, I'd pull the chassis and see if it fits.