Oh, I see what you want.
To do this models your panel in 3D using slabs, walls, etc. Select just the panel elements and view them in 3D. Via the parallel projection settings, set the view to be perpendicular to the face of the panel that is to be the top. Select everything (remember only the panel elements should be in the 3D view) and then choose Save Custom Component... This is found under File>Library and Objects in my work environment. The option you need here is Custom Object Component. Select this option and then click "Choose". The part will then save to the folder/directory you select with the name you give it. After a few seconds, you should see a dialog saying that the part is now available to object that recognize it.
If you go back to your cabinet and select a custom end panel the custom panel parameter will again appear with the name of the component you just created added to the list of available panels.
In my brief experimentation with one of the stock AC cabinets, this works fairly well, in that the panel you just created is indeed added to the cabinet. However, I found that the cabinet doors are then distorted and extra lines are added to the front/rear elevations that are not desired. Depending on your need this may be acceptable or not. Unfortunately, there is not an easy fix for this in the object.
I hope this helps you.
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K