Daz products have to be transferred using the Maya Bridge. Once in Maya, you can combine any assets you already have with your Daz products.
You can combine assets in Maya, but the tool brings in a new scene so it needs to be imported into the original Maya scene after.
By exporting the assets out of the scene that was created with the bridge and then imported into the Maya 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 Maya once it's open.
For more information on how to transfer assets from Daz Studio, refer to the article, “Maya Bridge: Exporting character(s) using Daz to Maya Bridge.”