Hi moujiik, try the PutOnGround script which you can find here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5427.13

It would not be difficult to make a separate version where you could pick an additional object for the level instead of z=0 like PutOnGround does. What should it do if the other object is not planar though, I guess align it to the minimum z ?

- Michael