iTween Move By is basically this, iirc, but otherwise...
You can use Get Position to grab the current position of the Object, say this is something like ( 20.23, 15, 19.8 ).
Now how much do you need to move it and in which direction? Make a vector that is that value, like (0, -2, 0).
Now add the two together, save it in a variable and that is your new position target, Tween to that. In this example it would be ( 20.23, 13, 19.8 )