You cannot add custom models to system prompts. However, you can clone them, to customize them however you like. This includes both adding/removing models, and changing the prompt text.
1. Clone the System Prompt
Go to prompts
Click on a system prompt
In the blue message box in the top, go to the right-hand side, click clone prompt
Note. If you don’t make any changes, and click off of the new prompt, it will disappear. So make sure you make the changes first.
You now have full customisability of the prompt. You can:
Delete models
Add new models
Clone models
Change text within the prompt instructions.
This applies to every system prompt, for all the interfaces (scene beat completion, scene summarisation, text replacement, and workshop chat).