Yes you can use a serial keypad from a Newton. It was a fairly common hack back in the say ... for what reason I cannot say. Though it is not plug-n-play.
http://en.wikipedia.org/wiki/Apple_Keyboard
Follow this link to see the details and pictures of how one would look working on an SE:
http://68kmla.org/forums/viewtopic.php?t=6840&start=25