Plesk really shouldn't have its core edited. When you need domain level config changes there's a file you need to edit outside that file. Under Apache that file was called vhost.conf
under the directory for your domain. It would then append that to the base config. It looks like nginx uses a similar process.
Based on this post here's what you need to do
Here steps how to add custom include in nginx virtual host config:
mkdir /usr/local/psa/admin/conf/templates/custom/domain
cp /usr/local/psa/admin/conf/templates/default/domain/nginxDomainVirtualHost.php /usr/local/psa/admin/conf/templates/custom/domain/
add in /usr/local/psa/admin/conf/templates/custom/domain/nginxDomainVirtualHost.php:
<?php if (file_exists($VAR->domain->physicalHosting->vhostDir . '/conf/nginx.conf')): ?> include <?php echo $VAR->domain->physicalHosting->vhostDir;?>/conf/nginx.conf; <?php endif ?>
/usr/local/psa/admin/bin/httpdmng --reconfigure-all # to apply new configuration for all domains
As result, if domain has conf/nginx.conf - it will be included into virtual host config.