Slightly longer answer -- you might be able to rig it with a sound capture progam so you got out the headphone jack what was coming in the mic, and then run a 3.5mm to 3.5mm stereo cable over to the MacPro. But why? If it is for recording, you'd be far and away better to record directly to a file on the portable, then move the file over.
But... Why go through all of this when you can get a real external mic cheaply enough?
May help a bit if you gave us some kind of inkling of what you are trying to accomplish.