New plugins are trivial to write and add to the installer - it just needs updating in a few places. See 

https://github.com/theforeman/puppet-foreman/blob/master/manifests/plugin/bootdisk.pp
https://github.com/theforeman/foreman-installer/blob/develop/config/foreman-installer.yaml#L38-L40
https://github.com/theforeman/foreman-installer/blob/develop/config/answers.yaml#L18

For an example. Obviously your new plugin class can have arbitrary puppet code in it, including creating `.ssh`