Just do what KyleSarnik said. Use a manned shilka with no ammo/fuel. Why waste time placing an empty vehicle and then moving guys into it?
But to answer the movincargo question:
{_x moveincargo vehiclename} foreach (units groupname)
or, if you want to use individually named units:
{_x moveincargo vehiclename} foreach [SS1,SS2,SS3,SS4,SS5] (in your case)
As you can see, it's a lot easier if they are in the same group.