Thresholds are typically metal, either aluminum or bronze. A pic or better description of what your asking about would help as I am puzzled because a threshold or sill would need to go in before or at the same time as the doors typically. And I would say sight unseen, a definite no to composite.

Thank you for your response! I had custom doors and the sill was not included and I was not advised to have it installed prior to frame installation. The frame is a heavy iron one rectangular piece that was fit into the door opening. Currently the frame is sitting on top of the stone and there is about 2" space under the frame. But the stone under the frame is wider than frame. So I need to compensate for the width difference on the edge.

That needed to go in before the door. It is called a sill and it helps to support the door and the threshold typically. I would not be happy that the installer of a custom door thought it was ok to leave this like that.

I know this is an old thread but was curious what you ended up doing. I have the same issue

One thing is to get the opinion of the door manufacturer. Send pictures to their Tech Support group.