Like this:
Put this the init field of the crate: [this,(getpos this),**PUT THE HEIGHT YOU WHANT HERE**] exec "height.sqs"
And the following in your mission folder:
height.sqs _object = _this select 0
_objpos = _this select 1
_height = _this select 2
_object setpos [(_objpos select 0),(_objpos select 1),(_objpos select 2) + _height]
exit