Nginx Install self signed certificate after vhost is created?

of course it's possible if you know how to do it manually

or just use official site's nginx vhost generator, enter domain name and check the self-signed ssl box to generate the instructions for manually doing it at Generate Centmin Mod Nginx Vhost - CentminMod.com LEMP Nginx web stack for CentOS

the generated nginx vhost is slightly different as it's for 123.08stable not 123.09beta01

or use official site generator for manual self-signed ssl setup but just use setup a dummy new nginx vhost with dummy domain and yes to self-signed ssl, then copy and paste that nginx domain.com.ssl.conf format only (not ssl files) itself to your existing site and search and replace the domain name and then remove the dummy new nginx vhost