Mongodb - sharding + replicating an existing collection
Yes, you can do it just like that. Shards in the cluster don't need to be replica sets, but it is of course recommended. And later, it is possible to convert those nodes to single node replica sets and after that add more nodes to those replica sets. However, after making node as replica set, you need to do little "under the hood" updates, so that config server (and mongos) will know that shard(s) are now replica sets and not single nodes anymore.