Here is my mod for adding hammocks. The game calls them "wool beds" because you can't change the names of furniture. Maybe this will give you some idea of what is required. I would change it for you to make stone bed, but I'm not a very good modder, so I may mess it up.
This goes in a file called "reaction_hammocks.txt."
reaction_hammocks
[OBJECT:REACTION]
[REACTION:SEW_LEATHER_HAMMOCK]
[NAME:Sew leather hammock]
[BUILDING:HAMMOCK_MAKER:NONE]
[REAGENT:A:1:SKIN_TANNED:NONE:NONE:NONE]
[PRODUCT:100:1:BED:NONE:GET_MATERIAL_FROM_REAGENT:A:NONE]
[SKILL:LEATHERWORK]
[REACTION:SEW_CLOTH_HAMMOCK]
[NAME:Sew cloth hammock]
[BUILDING:HAMMOCK_MAKER:NONE]
[REAGENT:A:10000:CLOTH:NONE:NONE:NONE]
[PRODUCT:100:1:BED:NONE:GET_MATERIAL_FROM_REAGENT:A:NONE]
[SKILL:CLOTHESMAKING]
Add these lines to the dwarf entity.
[PERMITTED_REACTION:SEW_LEATHER_HAMMOCK]
[PERMITTED_REACTION:SEW_CLOTH_HAMMOCK]
[PERMITTED_BUILDING:HAMMOCK_MAKER]
Put this at the end of building_custom.txt, or make another building file. Here is a link to a building designer:
http://www.bay12forums.com/smf/index.php?topic=54004[BUILDING_WORKSHOP:HAMMOCK_MAKER]
[NAME:Hammock Maker's Workshop]
[NAME_COLOR:7:0:1]
[WORK_LOCATION:2:2]
[DIM:3:2]
[BLOCK:1:0:0:0]
[BLOCK:2:0:0:0]
[TILE:0:1:179:179:179]
[TILE:0:2:179:179:179]
[COLOR:0:1:7:0:0:7:0:0:7:0:0]
[COLOR:0:2:7:0:0:7:0:0:7:0:0]
[TILE:1:1:0:205:185]
[TILE:1:2:32:32:32]
[COLOR:1:1:7:0:0:7:0:0:7:0:0]
[COLOR:1:2:0:0:0:0:0:0:0:0:0]
[TILE:2:1:219:205:185]
[TILE:2:2:176:176:176]
[COLOR:2:1:7:0:0:7:0:0:7:0:0]
[COLOR:2:2:7:0:0:7:0:0:7:0:0]
[TILE:3:1:15:205:185]
[TILE:3:2:176:176:176]
[COLOR:3:1:0:7:0:7:0:0:7:0:0]
[COLOR:3:2:7:0:0:7:0:0:7:0:0]
[BUILD_ITEM:1:NONE:NONE:NONE:NONE][BUILDMAT][WORTHLESS_STONE_ONLY][CAN_USE_ARTIFACT]