In order to ease the adoption of metallb, we must allow injecting a piece of frr configuaration after the one provided by metallb is rendered.
This allows quick stop gap solutions and workarounds to those customers that are affected by bugs.

The content of the overriding configuration must be agreed each time and confirmed under a support exception.

1.

Create a configmap as follows 

apiVersion: v1
kind: ConfigMap          
metadata:                
  namespace: metallb-system
  name: bgpextras        
data:                    
  extras: |              
    # EXTRA CONFIG
    # ON MULTIPLE LINES    

2.
3.

The extra lines are appended in the bottom of the configuration

This is borderline as a bug, but must be backported back to 4.12