I think that, for what you said, write o whole new plugin for TCP encryption may not be the best approach.
Have a look at the TCP Sampler component http://jmeter.apache.org/usermanual/component_reference.html#TCP_Sampler It says that "Users can provide their own implementation. The class must extend org.apache.jmeter.protocol.tcp.sampler.TCPClient." Checkout JMeter`s source code and take a look at TCPClientImpl, BinaryTCPClientImpl, LengthPrefixedBinaryTCPClientImpl classes. Writing your own TCP Encrypted client would be a better way to solve your request and will take less code to put it on. Before start digging JMeter`s source code, try to write a TCP with SSL/TLS using a BeanShell sampler. Some resources related to this: http://stackoverflow.com/questions/6553074/secure-tcp-connections-in-java http://bytes.com/topic/java/answers/848845-java-socket-programming-aes-encryption http://pt.slideshare.net/sagarmali2012/securing-tcp-connections-using-ssl
