Daz products have to be transferred using the Unity Bridge. Once in Unity, you can combine any assets you already have with your Daz products. The bridge tool defaults to a new scene; therefore, the original Unity scene will need to be imported afterwards.
By exporting the assets out of the scene that was created with the bridge and then imported into the Unity scene, they can be combined. Use copy and paste if both the scenes are opened or by exporting the objects and importing in the original scene in Unity once it's open.
For further instructions on how to transfer assets, refer to the article, "Unity Bridge: How to use the Unity Bridge."