'gunnr,
IMHO it's best to leave 'dimpled' Express guns alone. The plastic piece the dimples hold in place retains the magazine spring, and also keeps the magazine cap tight. Now if you're never going to use the gun as a sporting gun again, I suppose it would be no big loss to drill out the dimples and negate the use of the magazine cap detent. But it will be necessary to jury-rig some way of keeping the magazine cap tight if you ever go back to the standard magazine.
If you want an Express with a magazine extension, best bet IMHO is to buy the factory version from the gitgo instead of trying to jackleg something. You get a good magazine extension into the bargain that way. Or buy a used, older version of the Express that doesn't have dimples to start with.
http://www.remington.com/products/fi...ic_7-round.asp
Best yet, learn to load the gun on the fly. "No magazine is ever big enough." - Louis Awerbuck, on shotguns.
JMHO, YMMV....
lpl