I haven't used the multisearcher personally, so I'll let others chime in. And I know nothing about the IndexMergeTool, I've only seen the interface in the Lucene Javadoc. And I must say the documentation isn't real helpful :(.
To add to an existing index, just instantiate the IndexWriter with the boolean create parameter set to 'false'. There's no contention problem with one (or more) searchers searching an index while it's being modified. HOWEVER, your searchers won't see the data you're adding UNTIL you close and re-open the readers/searchers. Best Erick
