Hi - HCP lets you build entirely custom applications from scratch and then link it to SuccessFactors using extensions. However, extending only some part of it to Onboarding solution would mean other API's or extensions in particular. At the moment I do not think there is such level of extensions supported.
Based on my experience, if the majority of your requirements can be met by HCP and only some of it in Onboarding, then my suggestion would be to have entirely a custom app on HCP and simply connect it to SuccessFactors. Clubbing SF-Onboarding and HCP would be cumbersome and a challenging task.
This is one of the growing demands from clients right now and I will keep adding my thoughts and inputs as and when I have them.
Stay tuned at KnockSf.