Yes yes and yes.
Yes you can install an Airport card - tons of them for dirt cheap on eBay.
But even simpler --- I take it your Mac Pro is connected via wifi to your network? --- if so under System Preferences > Sharing, just enable Internet Sharing from Airport, to Ethernet. Then just connect your PowerMac to to the Mac Pro via ethernet and it should all work perfectly fine.
Obviously this will work only when your Mac Pro is turned on -- but if it always is when you're using the PowerMac, no need to spend extra money on something you can do for free.