Under the assumption that expanding index means adding new shards, then the answer is that it is not possible to create new shards without a reindexing. This is because sharding is done using hashing and adding another bucket (shard) requires the hash function to be changed in order for new items into the shard.
As stated in http://wiki.apache.org/solr/SolrCloud#Re-sizing_a_Cluster it is a feature on the road map.