how to add shard into current cluster without sharing any chunks
There is other solution, if you want to migrate you data to new machine "on-the-fly" and have only short interruption when you transfer your clients to use that new machine.  With this you can replicate data to new location

If you want to add new shard to cluster and still keep balancer running... you can always use TAGs.. Tag your current shards and define that $MinKey - $MaxKey chunk area belongs to that TAG. Check here what I'm talking about