Unfortunately Impala queries are submitted to a different service endpoint than Hive queries, so you can't use WebHCat to submit queries to Impala.
If you're curious, here's a bit more information about how to submit queries to Impala. First, read the Impala Concepts and Architecture documentation. As you now know, you can submit your query to any node running the impalad daemon. The interface exposed by this daemon is specified in ImpalaService.thrift. There are a number of open source clients that have been implemented that will allow you to submit queries to Impala from the command line, from a web interface, or from a library in your favorite programming language. Here are a few examples:
- impala-shell: command-line interface that ships with Impala
- Impala app in Cloudera Hue: web interface
- impyla: Python library
- impala-ruby: Ruby library
- php-impala: PHP library
- ImpalaSharp: C# library
- impala-java-client: Java library