You may need to customize a configurationfile for eg. an openshift-router or the registry.
If the dc supports the TEMPLATE_FILE environment, you can do it in three steps, otherwise you should find
a hook to mount the file in an expected location.

First get the original configuration file and modify it as desired. In this example, we are increasing the maximum allowed connections.

 # oc rsh router-xxx cat /var/lib/haproxy/conf/haproxy-config.template > haproxy-config.template
 # vim haproxy-config.template  # modify as desired, eg.

--- /var/lib/haproxy/conf/haproxy-config.template       
+++ /var/lib/haproxy/conf/custom/haproxy-config.template       
@@ -7,6 +7,7 @@
 {{ $workingDir := .WorkingDir }}
 global
   # maxconn 4096
+  maxconn {{env "ROUTER_MAX_CONNECTIONS" "20000"}}
   daemon
 {{ with (env "ROUTER_SYSLOG_ADDRESS" "") }}
   log {{.}} local1 {{env "ROUTER_LOG_LEVEL" "warning"}}
@@ -39,6 +40,7 @@

 defaults
   # maxconn 4096
+  maxconn {{env "ROUTER_MAX_CONNECTIONS" "20000"}}
   # Add x-forwarded-for header.
 {{ if ne (env "ROUTER_SYSLOG_ADDRESS" "") ""}}
   option httplog

1- create a configmap from your new template file, eg.
2- reference the new file via the TEMPLATE_FILE environment if supported
3- use the volume feature to mount the configmap as a file

 
 # oc create configmap router-haproxy-34 --from-file=haproxy-config.template
 # oc set env dc/router TEMPLATE_FILE=/var/lib/haproxy/conf/custom/haproxy-config.template
 # oc volume dc/router --add --overwrite     \
      --name=config-volume     \
      --mount-path=/var/lib/haproxy/conf/custom     \
      --source='{"configMap": { "name": "router-haproxy-34"}}'

Now verify and rollout the new config.

 oc describe dc router
 oc rollout latest router