It should be no problem at all to add sidemarkers. You could drill through the fender and put it in. Just make sure to have a sealant like clearcoat so the thing doesn't rust. Don't get 2, 1 will be just fine.
<span style='font-size:8pt;line-height:100%'><span style='color:blue'>1998 BMW 323is Coupe, Black
18" M3 Hyper Silver Rims
2 12" Rockford Fosgate Subs; Sealed
1.0 Farad Capacitor
800 Watt 2-Way Rockford Fosgate Amplifier</span></span>