Most SD 6-string humbuckers can be ordered with a nickel cover for an extra charge. I'm pretty sure the only exceptions are the Invader, the Full Shred, the Screamin' Demon, the Stag Mag, the Dimebucker, and the Parallel Axis models ... in short, anything with non-standard pole pieces.
Yes...that's what I tend to do too. If a guy wants a pickup with a nickel cover and he doesn't want to wait for a custom order I keep nickel covers in stock and just install them for him. Not sure if it voids the warranty though and 21 day exchange...I think it probably would unless the cover was originally installed by Duncan Pickups. Lew