You should have a separate DB for your website and one for Oxwall.  There is no bridge currently available to share the two.  So your user will need to sign up to both websites.

If you want to have what they call a SSO  Single Sign On then you will need to either code it yourself or hire a professional to do so. 

Short of that you can  just place your site link using custom html option to share your site  link on the main page and have them register again on your new site.