Hello - Behance and ProSite are synced, so you have to create your projects on Behance, and then you can come to ProSite to use them and customize your site. So no, you cannot use ProSite without Behance. However, if you dont' have to have public projects on your Behance, you can make them private, but they'll still be available for you to use on ProSite.
Hope this helps! Find more at help.behance.com