Chen,
Is HttpClient the external component you're trying to use?
If so, GEP ships with HttpClient 4.2. To use the version that ships with GEP, add the following bock of XML to your pom.xml under /project/dependencies node:
<dependency>
<groupId>org.apache.httpcomponents</groupId>
<artifactId>httpclient</artifactId>
<version>4.2</version>
</dependency>
Rebuild your jar file using mvn and redeploy the jar file. That should allow you to use HttpClient.
If you need a more recent version of HttpClient, then you'll need to drop all of the HttpClient jars and its dependencies into the product's deploy folder (C:/Program Files/ArcGIS/Server/GeoEventProcessor/deploy/).
Let us know if this gets you any further along.
-Javier