How can I use the key instead of to change field focus?

I am using Acrobat 9 Pro.

We have a fillable PDF where the users enter numbers into fields.  They want to be able to use the <enter> key on the 10-key pad instead of the <tab> key on the keyboard to move the focus to the next data entry field.

How can I set my PDF so that it recognizes the <enter> key instead of the <tab> key for changing focus?

Thanks!