... ghost-ring sights on a Benelli M1S90 without screwing up the barrel and without buying a new barrel. I don't know how the rear rifle sight is mounted on the barrel. Does anyone know if this is a possible conversion?

I assume it would not be a huge issue to have a smith mount the rear ghost-ring onto the receiver.

The only M1S90’s I’ve had experience with had 14 inch barrels; I’m assuming they were similar to yours except for the barrel length (I don’t guess yours is that short).
 
Should be doable.  

However, the front sight is probably too low for a ghost ring - so it will have to be removed and replaced.  I’m pretty certain it’s silver-soldered on, so removing it may damage the finish.  Thus you may need to reblue the barrel.

The rear sight itself is held in its dovetail by two incredibility small hex screws.  I’m pretty sure the rear sight base, like the front sight, is silver-soldered on.  The base could be left alone.  

However, if you’re going to remove the front sight and reblue the barrel anyway, it makes sense to get rid of the base at the same time.

Mounting the ghost ring rear sight is the easy part.

Upgrading to a ghost ring sight system (maybe with a tritium front sight) is justified, even if it costs a bit, when you consider what a really fine shotgun an M1S90 is.

Yeah, 199 is right.

Onmy GUNSITE Combat Shotgun tabe they show a clip of their smithy, they silver solder the bigger front sights up front then repark the barrel, the rear is tapped on the reciever, IIRC.

Radius is high enough that that rear stub rifle sight is not going to be in the way, but it could be removed at the same time.

Might want to consider the Ashley Express sights (if he is still doing them for the Benelli).  The sights used the factory cut for mounting.  Superb sighting system for a shotgun, BTW.