This feature is not yet available in Pharo Launcher.
We are working to provide a configuration per image / template where you could describe a specific vm to run your image or a script to run at the image creation to load packages, etc.
For now, what you could do, is to have a CI job preparing the image with all you need for you, zip it, and use it as a template from Pharo Launcher. It is already doable from https://ci.inria.fr/pharo-contribution/. If your project is private, then you could add a source to a private Jenkins server.
I am not sure which executable to run with this that will execute the same executable as pharo launcher.
Pharo Launcher stores its vms under $HOME/Documents/Pharo/vms
If you want to run the same VM as PharoLaucher, you just need to know the Pharo version of the image (e.g. 70) and the architecture (e.g. 64 bits).
Then you can run you image with $HOME/Documents/Pharo/vms/70-x64/Pharo.app/Contents/MacOS/Pharo Pharo.image ../scripts/setup-client-application.st (the end of the path on windows or linux to the executable will change a bit)