Travis, you CAN make monsters move. Just add to object property X +1 and the monster will move one meter in the X direction. Use a combination of those to make it move. You can use this to make it wander or move toward a fixed point, but it will not chase a moving point like a player ship. To get it to chase a player ship you would need a LOT of code, accounting for numerous positions of the monster and the players.
Another way to get a monster to chase players is to constrain the monster to a generic object (constantly copy object property X,Z of the generic to the monster). Then tell the generic object to move toward the player at whatever speed you like. This will work fine, but Science will scan the generic object so it won't look like a regular monster. Give it a clever name and unique map icon color.
You cannot put any vessel data files in the mission folder at present. I expect it in a future upgrade. But I have a workaround for your asteroid base:
Build a normal base. Call it "Asteroid Base." Set it's shields to 2000.
Build a generic object on top of the base. Give it the asteroid DXS and texture files. Set pushRadius to zero and artscale to be slightly larger than the base. You might need to move it one meter on any axis to assure that it doesn't repel the base. For added realism you can give the generic object a pitchDelta, rollDelta, and angleDelta so your asteroid base appears to be slowly turning in space.
You could even have the base defend itself by putting a neutral destroyer inside the base with its engines disabled. just remember that Science will see that there is a USFP Destroyer inside the asteroid base.
For more ideas, look at the way we used generic objects in the mission "Party Crashers."
"Damn the torpedoes! Four bells, Captain Drayton!"
(Likely actual words of Admiral David Farragut, USN, at the battle of Mobile Bay. Four bells was the signal for the engine room to make full steam ahead).