Okay, I found where I've seen this before, it's done in the Eagle Takes Flight campaign (I ripped this code straight from the -shp.tbm file of that campaign):
;meteor with wrong ship type. sexps using cruisers are not valid with
;this ship in the mission. May affect ai. This is a booby trapped asteroid,
;it has a propulsion system and can be used like a ship. it has no weapons,
;but you can ram things with it.
$Name: Asteroid#meteor
$Short name: meteor
$Species: Terran
$POF file: asteroid01.pof
$Detail distance: (0, 100)
$Show damage: NO
$Density: 1
$Damp: 0.0001
$Rotdamp: 0.0001
$Max Velocity: 0.0, 0.0, 80.0
$Rotation time: 0.0001, 0.0001, 0.001
$Rear Velocity: 0.0
$Forward accel: 1.0
$Forward decel: 1.0
$Slide accel: 0.0
$Slide decel: 0.0
$Expl inner rad: 50.0
$Expl outer rad: 500.0
$Expl damage: 100.0
$Expl blast: 20.0
$Expl Propagates: NO ;; If set to Yes, then when the ship dies, the explosion propagates through it.
$Shockwave Speed: 500.0 ;; speed shockwave expands at, 0 means no shockwave
$Default PBanks: ()
$Default SBanks: ()
$SBank Capacity: ()
$Shields: 0
$Power Output: 1.0
$Max Oclk Speed: 80
$Max Weapon Eng: 0.0
$Hitpoints: 1000
$Flags: ( "cruiser" )
$AI Class: NONE
$Afterburner: NO
$Countermeasures: 0
$Scan time: 4500
$EngineSnd: -1 ;; Engine sound of ship
$Closeup_pos: 0.0, 1.0, -250
$Closeup_zoom: 0.5
Put that text into a notepad file, save it as yourmodname-shp.tbm (replace yourmodname with whatever your campaign is called), and put it under data/tables in your mod's folder (I assume you've got a mod.ini set up. If not, do that first). Run FRED with your mod and you should be able to ctrl-click the rock just like any other ship. Set it as a kamikaze ship, and you've got your ramming rock! If you encounter any problems post here. Either I or someone else will no doubt help you out.
