Haproxy % traffic to a cluster

This can be achieved by having haproxy connect to itself, giving you two tiers of load balancing.

The first listen uses the balance source option and server weights to split the traffic between your existing server and the cluster. The second tier uses balance roundrobin with no persistence to rotate incoming connection among cluster members.

Listen 10.0.1.1:80
    Balance source
    Server oldserver 10.0.1.10 weight 90
    Server newcluster 10.0.1.20 weight 10

Listen 10.0.1.20:80
    Balance roundrobin
    Server cluster1 10.0.1.31
    Server cluster2 10.0.1.32
    Server cluster3 10.0.1.33