The Frontline "spinner" is only a spinner above the panel. It's really a joystick underneath. You turn the knob up top, and it rotates an actuator on the bottom that depresses microswitches, just like a joystick actuator would. That's why MAME has the aiming controls mapped to joystick 2 for Frontline. (and Wild Western, The Tin Star, Sherrif, and Bandido)
I'm not aware of any way to make it work with a regular spinner. Your best bet is either a) find an original controller; or b) use a 2 joysticks with a top-fire button on each.
edit: Darn you Spidermonkey, you posted while I was typing!