This is what i found, This will help you make your own prefabs. located right in the manual
5.1.1 How To Make Your Own Prefabs
You can use the map editor to create your own prefabs. You must follow the
steps below to ensure your prefabs are created correctly.
1. First click the File/New menu item to begin a brand new map - you must
start from scratch when creating prefabs.
2. Select the segments you wish to use and paint them to the top left corner
of the map. You can use any combination of segments such as rooms,
corridors, doors and windows. You must not use entities from the entities
library for your prefab design.
3. You can design your prefab over several layers. We recommend a height
of no more than five layers, beyond this and it becomes difficult to place
such a tall structure from the top/down perspective.
4. When you have painted your segments, position the 3D cursor at the
lowest layer, at the top left corner.
5. Now press “E” and select the entire area of segments that define the
bounds of your desired prefab. You can use the - and + keys to move up
and down the levels.
6. Once the whole prefab is surrounded by the copy selector, press CTRL+C
to copy the selection into the clipboard.
7. Now press CTRL+J to launch the “save prefab” dialog. Choose a location
and enter a name for your prefab remembering to end your filename with
“.FPP”.
8. After the dialog, you will be presented with a screen where you can
preview the prefab and zoom around with the camera using the W,A,S,D
keys.
9. When you are happy with the view of the prefab, click the left mouse
button to take a snapshot of the prefab. This image will be used as the
thumbnail for the prefab.
10. You can now select the Add new prefab icon and you will find your prefab
in the location you saved it to.
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