Add Embed To Cast Unit
When you have your cast unit in your model, change your selection tool to Select Assemblies.
With Select Assemblies activated, choose your desired embed and place it in the Cast Unit.
With Select Assemblies still activated, select the embed and then right click on the embed. From the right click menu, navigate down to the assembly option and choose Add as Sub-Assembly.
You to select the assembly that you want to add the sub-assembly to from the status bar on the bottom left.
The embed will now be part of the overall cast unit. To double check this, choose Select Components as your selection tool, right click on cast unit and navigate to inquire, then cast unit.
You will see the cast unit itself is pink, signifying it is the main part of the cast unit and the embed is the sub-assembly is represented as yellow. You can see this information listed in the inquire box as well.