Tough to answer, there is so much to it.

JSF pages must interact with the FacesServlet.  If it doesn't you can
use your plain JSP pages*.  You can migrate your JSP to JSF page by
page that way.

*JSF actually can use JSP pages or XHTML pages or any other type of
page really. For the sake of answering your question, just ignore this
footnote. ;)

(Sorry it's a little old)
You can perfectly mix JSP and JSF, specially if the functionality is
unrelated
(like add new pages using JSF to an application)
I even included JSF segments in an big JSP, but you need to be careful.
Good luck