Hi Bill,
Im not sure what exactly are you trying to achieve with this. Say you have a Kentico site and some different site which does not use Kentico, right? What do you want the non-Kentico site to work like? If you want to share javascripts/css files than use what Brenden has offered. If you want to get News from Kentico then use REST services to get the content from Kentico.
I-frames would just simply load your Kentico site into non-Kentico site, I dont think this is what you are looking for.
Let us know if you have further questions.
Kind regards,
Richard Sustek