Need help with number format and custom format script

Can someone tell me how to have a number format and a custom format script? I am using this script to put the feet hash mark when I enter a number. Like so,

AFNumber_Format(0,0,0,0,"\'",false);

The thing is I cannot set the number format so I can only enter a number if I use this script. If I don't use this script then I can set the number format but then I lose the feet hash mark. I can only use one or the other formats. Is there a way I can have both? At the moment I only have this script in the custom format script pane window. So then, I can enter text in the field. Not what I want to do. Can someone help? Thanks.