Some people use this thing called the *wiggle* method. You'll have to wait for someone to answer more clearly though. I don't know how to explain it.
The "Geomerging item" is whatever item you wish for it to be. Use the first part of this guide, otherwise known as the old method: http://www.forgehub.com/forum/forging-101/1722-merging-immovable-objects-map-geometry.html You could also try the Wiggle method, but just like Y35 above me, I have no idea what it is.
There isn't anything like the door in Foundry. You have to use the old method, which is all I ever used, since I didn't know how the door method worked...stupid, I know... The link to the old method can be found in Bluejayfan's post.
Indeed, that old guide is likely the best way to go about it. It means that geomerging will take a lot more time, as you can only really effectively set up one or two merges per round. There's also the aforementioned "wiggle method" that I'm unfamiliar with, along with a save-and-quit method that was evidently well-known in some circles of Foundry-forgers just prior to Sandbox's release. Unfortunately, I'm unfamiliar with that one, either (though I have a guess as to how it's done).