Daz products have to be transferred using the Unreal Bridge. Once in Unreal, you can combine any assets you already have with your Daz products; however, this will also import as a new scene.
By exporting the assets out of the scene that was created with the bridge and then imported into the Unreal 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 Unreal once it's open.
For further instructions on how to transfer assets, refer to the article, “Unreal Bridge: How to use the Unreal Bridge.”