i suggest looking at forging 101 called merging objects into the map geometry. but i find it easier to do this method. spawn immovable object (like double box or fence box etc.) and surround it with other immovable objects. i like to surround it with fence walls though. then either spawn 1 door and flip it over. then place it in the middle. go directly above the object you want to merge and hold it (press A) and either wait until it stabilizes or save and quit. you can also use 3 doors by placing 1 door on left side, 1 door in middle, and 1 door right side.
http://www.forgehub.com/forum/forging-101/1722-merging-immovable-objects-map-geometry.html this should be in the "halo discussion" thread. kthnx.
Here It may seem a little complicated to understand, but the key to learning it is experimentation, i recommend trying the 'door' method first.