Commercial radiation therapy treatment planning systems have quite rigid workflows, so they cannot import arbitrary images volume and paste it into a patient image. The optimal solution depends on what exactly you would like to achieve (are you interested more in the patient dose or how much radiation the pacemaker is exposed to, how accurately you want to model the pacemaker and its placement under the skin, if you have easy access to a radiation treatment planning system, etc.).
In general, you If you want to create a simulated image of a patient CT with a pacemaker then the best is to start with a real CT image of a patient and a real CT image of a pacemaker and combine them.
You might start with simply positioning the pacemaker image (using Transforms module) and add it to the patient image (using Add Scalar Volumes module).
If you want to be more realistic then you can warp the patient image to make space for the pacemaker. You can do this by image warping using fiducial points. You can do this by using Fiducial registration wizard module.
If you want to create simulated CT image of a pacemaker from a CAD model then you need to know the radiometric density of every component of your device. The easiest is to acquire a CT scan of it, but if that’s not an option then you can measure typical densities from CT scan of another pacemaker and then use that when you create your own image. You can create the image by exporting each piece of the device as a separate model file, load them into Slicer and import them into a segmentation. Then fill the image with the chosen radiometric density value using Mask volume effect in Segment Editor.