yeh u can put after market gear knobs to a auto my mate did one the other day he put a HSV leather gearknob looks pretty nice and felt good to. im not sure bout if its legal but thats for mouce to tell ya.
So long as it's secure and fully functional without altering the safety aspects of the shifting pattern (such as you still need to have to push the button to get into and out of "P"), there's nothing that states it's ILLEGAL to fit an aftermarket handle.
That's the good news.
The bad news is that MILD50 was right, there are two different types of 'action' to change gears, one pushes a locking shaft when you push the button, and one pulls the locking shaft when you push the button.
I believe that it's possible to modify them, but then you start getting into some grey legal areas, you'd be better off just getting one that's going to work straight off the shelf.
Bite off more than you can chew, and chew like hell - Peter Brock (1945-2006)