You may insert by snapping the fixture you wish to any of the bars of a truss simply by choosing the most appropriate view.
e.g. if you have two fixtures hanged it the same location in the front view, select the one you wish to have on the top bar of the truss and in the right view move it until it snaps to the upper bar.

Hi ! What if you want to overhang a fixture onto a pipe, so that the hook is under the fixture ?

For this case, you need to select the fixture, move the anchorage to the desired position (e.g. the snapping point of the fixture) and rotate the fixture in the view that better fits your needs.